Fix fragment shader position references

[rileyjshaw]

Fragment Shaders: The Basics states that xy coordinates are relative to the top-left of the buffer. Really, they default to the bottom-left. From OpenGL:

By default, gl_FragCoord assumes a lower-left origin for window coordinates and assumes pixel centers are located at half-pixel centers. For example, the (x, y) location (0.5, 0.5) is returned for the lower-left-most pixel in a window. The origin of gl_FragCoord may be changed by redeclaring gl_FragCoord with the origin_upper_left identifier.

[rileyjshaw]

Ping @mikolalysenko - have you seen this?