Rework of filter boundary behaviour

[Quickmarble]

This extends the work started in #769 (specifying boundary behaviour with Border<P>) to filter*() functions and friends. Refactoring all of them while keeping track of performance might take a while.

Closes #2.

[Quickmarble]

There might be more places where boundary behaviour should be reviewed, but I guess it's enough changes for the scope of this PR. I need to update docs and then it'll be ready.

The following functions have been modified to account for Border<P>:

• filter(), filter_parallel(), gaussian_blue_f32(), separable_filter(), separable_filter_equal(), filter_clamped(), {horizontal,vertical}_filter(), laplacian_filter(), laplacian_filter_parallel();
• sharpen3x3(), sharpen3x3_parallel(), sharpen_gaussian();
• bilateral_filter();
• box_filter();
• median_filter();
• gradients_grayscale(), gradients(), {horizontal,vertical}_{sobel,scharr,prewitt}(), {sobel,prewitt}_gradients();
• cell_histograms();
• {row,column}_running_sum().

One- and two-dimensional filters now employ BoundaryAccess::convolve_at() and BoundaryAccess::convolve_{horizontal,vertical}_at().

[Quickmarble]

Benchmarks before:

test edges::benches::bench_canny                                                      ... bench:   1,546,706.70 ns/iter (+/- 22,128.71)
test filter::benches::bench_filter_clamped_gray_3x3                                   ... bench:     584,080.35 ns/iter (+/- 1,641.65)
test filter::benches::bench_filter_clamped_gray_3x3_ref                               ... bench:     156,551.45 ns/iter (+/- 161.06)
test filter::benches::bench_filter_clamped_gray_5x5                                   ... bench:   1,266,857.00 ns/iter (+/- 8,958.98)
test filter::benches::bench_filter_clamped_gray_7x7                                   ... bench:   2,244,828.10 ns/iter (+/- 23,081.02)
test filter::benches::bench_filter_clamped_parallel_gray_3x3                          ... bench:     253,829.89 ns/iter (+/- 353,977.36)
test filter::benches::bench_filter_clamped_parallel_gray_5x5                          ... bench:     342,850.21 ns/iter (+/- 83,774.43)
test filter::benches::bench_filter_clamped_parallel_gray_7x7                          ... bench:     513,707.80 ns/iter (+/- 94,758.57)
test filter::benches::bench_filter_clamped_parallel_rgb_3x3                           ... bench:     305,939.88 ns/iter (+/- 75,138.22)
test filter::benches::bench_filter_clamped_parallel_rgb_5x5                           ... bench:     486,178.99 ns/iter (+/- 74,637.19)
test filter::benches::bench_filter_clamped_parallel_rgb_7x7                           ... bench:     738,947.28 ns/iter (+/- 113,300.36)
test filter::benches::bench_filter_clamped_rgb_3x3                                    ... bench:     983,044.05 ns/iter (+/- 10,010.20)
test filter::benches::bench_filter_clamped_rgb_5x5                                    ... bench:   2,349,108.60 ns/iter (+/- 30,522.30)
test filter::benches::bench_filter_clamped_rgb_7x7                                    ... bench:   4,226,682.10 ns/iter (+/- 30,645.08)
test filter::benches::bench_gaussian_f32_stdev_1                                      ... bench:     120,661.23 ns/iter (+/- 1,003.70)
test filter::benches::bench_gaussian_f32_stdev_10                                     ... bench:     750,532.90 ns/iter (+/- 15,513.70)
test filter::benches::bench_gaussian_f32_stdev_3                                      ... bench:     262,145.30 ns/iter (+/- 1,175.09)
test filter::benches::bench_horizontal_filter                                         ... bench:     619,759.00 ns/iter (+/- 1,090.36)
test filter::benches::bench_separable_filter                                          ... bench:     447,399.10 ns/iter (+/- 986.46)
test filter::benches::bench_vertical_filter                                           ... bench:     619,702.70 ns/iter (+/- 1,900.86)
test filter::bilateral::benches::bench_bilateral_filter_greyscale                     ... bench:  23,385,243.60 ns/iter (+/- 20,811.49)
test filter::bilateral::benches::bench_bilateral_filter_rgb                           ... bench:  25,062,125.20 ns/iter (+/- 163,893.23)
test filter::box_filter::benches::bench_box_filter                                    ... bench:     656,247.15 ns/iter (+/- 2,190.00)
test filter::median::benches::bench_median_filter_s100_r1                             ... bench:      38,876.62 ns/iter (+/- 604.49)
test filter::median::benches::bench_median_filter_s100_r4                             ... bench:      72,981.55 ns/iter (+/- 652.40)
test filter::median::benches::bench_median_filter_s100_r8                             ... bench:     121,941.80 ns/iter (+/- 773.93)
test filter::median::benches::bench_median_filter_s100_rx1_ry4                        ... bench:      40,694.45 ns/iter (+/- 881.24)
test filter::median::benches::bench_median_filter_s100_rx1_ry8                        ... bench:      40,419.63 ns/iter (+/- 328.81)
test filter::median::benches::bench_median_filter_s100_rx4_ry8                        ... bench:      70,971.05 ns/iter (+/- 905.82)
test filter::median::benches::bench_median_filter_s100_rx8_ry1                        ... bench:     117,630.32 ns/iter (+/- 1,359.44)
test gradients::benches::bench_sobel_gradients                                        ... bench:   3,758,538.50 ns/iter (+/- 38,712.83)
test hog::benches::bench_hog                                                          ... bench:     303,231.67 ns/iter (+/- 864.03)
test integral_image::benches::bench_column_running_sum                                ... bench:         311.97 ns/iter (+/- 3.76)
test integral_image::benches::bench_row_running_sum                                   ... bench:         191.78 ns/iter (+/- 2.25)
test seam_carving::benches::bench_shrink_width_s100_r1                                ... bench:     147,886.17 ns/iter (+/- 344.99)
test seam_carving::benches::bench_shrink_width_s100_r4                                ... bench:     583,191.40 ns/iter (+/- 2,128.95)
test seam_carving::benches::bench_shrink_width_s100_r8                                ... bench:   1,145,089.20 ns/iter (+/- 19,530.64)

Benchmarks after:

test edges::benches::bench_canny                                                      ... bench:   1,606,450.30 ns/iter (+/- 13,825.03)
test filter::benches::bench_filter_clamped_gray_3x3                                   ... bench:     618,154.45 ns/iter (+/- 3,379.63)
test filter::benches::bench_filter_clamped_gray_3x3_ref                               ... bench:     156,640.23 ns/iter (+/- 684.63)
test filter::benches::bench_filter_clamped_gray_5x5                                   ... bench:   1,323,662.10 ns/iter (+/- 6,155.14)
test filter::benches::bench_filter_clamped_gray_7x7                                   ... bench:   2,320,877.20 ns/iter (+/- 17,540.28)
test filter::benches::bench_filter_clamped_parallel_gray_3x3                          ... bench:     250,470.22 ns/iter (+/- 375,711.28)
test filter::benches::bench_filter_clamped_parallel_gray_5x5                          ... bench:     336,580.32 ns/iter (+/- 68,551.22)
test filter::benches::bench_filter_clamped_parallel_gray_7x7                          ... bench:     496,986.41 ns/iter (+/- 72,033.12)
test filter::benches::bench_filter_clamped_parallel_rgb_3x3                           ... bench:     301,910.75 ns/iter (+/- 61,504.24)
test filter::benches::bench_filter_clamped_parallel_rgb_5x5                           ... bench:     467,288.24 ns/iter (+/- 63,345.47)
test filter::benches::bench_filter_clamped_parallel_rgb_7x7                           ... bench:     710,636.18 ns/iter (+/- 107,407.17)
test filter::benches::bench_filter_clamped_rgb_3x3                                    ... bench:   1,014,798.80 ns/iter (+/- 15,952.69)
test filter::benches::bench_filter_clamped_rgb_5x5                                    ... bench:   2,472,048.90 ns/iter (+/- 23,803.58)
test filter::benches::bench_filter_clamped_rgb_7x7                                    ... bench:   4,440,285.70 ns/iter (+/- 89,498.21)
test filter::benches::bench_gaussian_f32_stdev_1                                      ... bench:     125,662.53 ns/iter (+/- 644.16)
test filter::benches::bench_gaussian_f32_stdev_10                                     ... bench:     794,682.10 ns/iter (+/- 29,472.12)
test filter::benches::bench_gaussian_f32_stdev_3                                      ... bench:     266,603.67 ns/iter (+/- 586.30)
test filter::benches::bench_horizontal_filter                                         ... bench:     621,516.65 ns/iter (+/- 1,472.73)
test filter::benches::bench_separable_filter                                          ... bench:     449,685.30 ns/iter (+/- 2,281.07)
test filter::benches::bench_vertical_filter                                           ... bench:     621,339.20 ns/iter (+/- 6,215.51)
test filter::bilateral::benches::bench_bilateral_filter_greyscale                     ... bench:  17,407,408.50 ns/iter (+/- 94,707.10)
test filter::bilateral::benches::bench_bilateral_filter_rgb                           ... bench:  25,102,466.00 ns/iter (+/- 30,795.66)
test filter::box_filter::benches::bench_box_filter                                    ... bench:     656,730.50 ns/iter (+/- 6,125.16)
test filter::median::benches::bench_median_filter_s100_r1                             ... bench:      36,239.47 ns/iter (+/- 166.48)
test filter::median::benches::bench_median_filter_s100_r4                             ... bench:      72,511.96 ns/iter (+/- 1,528.24)
test filter::median::benches::bench_median_filter_s100_r8                             ... bench:     120,008.00 ns/iter (+/- 1,108.74)
test filter::median::benches::bench_median_filter_s100_rx1_ry4                        ... bench:      37,737.98 ns/iter (+/- 212.96)
test filter::median::benches::bench_median_filter_s100_rx1_ry8                        ... bench:      39,023.13 ns/iter (+/- 260.45)
test filter::median::benches::bench_median_filter_s100_rx4_ry8                        ... bench:      71,151.80 ns/iter (+/- 1,219.23)
test filter::median::benches::bench_median_filter_s100_rx8_ry1                        ... bench:     118,220.14 ns/iter (+/- 1,505.95)
test gradients::benches::bench_sobel_gradients                                        ... bench:   3,942,084.35 ns/iter (+/- 15,059.41)
test hog::benches::bench_hog                                                          ... bench:     304,469.85 ns/iter (+/- 1,022.83)
test integral_image::benches::bench_column_running_sum                                ... bench:         312.00 ns/iter (+/- 3.12)
test integral_image::benches::bench_row_running_sum                                   ... bench:         190.43 ns/iter (+/- 0.97)
test seam_carving::benches::bench_shrink_width_s100_r1                                ... bench:     155,330.17 ns/iter (+/- 389.11)
test seam_carving::benches::bench_shrink_width_s100_r4                                ... bench:     612,569.00 ns/iter (+/- 3,089.73)
test seam_carving::benches::bench_shrink_width_s100_r8                                ... bench:   1,202,706.30 ns/iter (+/- 4,013.73)

[Quickmarble]

Should be ready now!