libsixel: update to 1.8.7.

See the following release page about the current project status:
 https://github.com/saitoha/libsixel/releases/tag/v1.8.7

Upstream changes (from NEWS):

                  ------------------------------
                   What's new in libsixel-1.8.7
                  ------------------------------

  for a comprehensive view of security-fix progress, see the page below
  saitoha/libsixel#203

* fix invalid pointer access in encoder.c (#193, #195)
  Thanks to @momo-trip, @akinomyoga

* fix wrong HLS to RGB conversion. (#191)
  Thanks to @gnachman, @j4james

* fix NULL pointer dereference problem in img2sixel.c (#192)
  Thanks to @momo-trip, @akinomyoga

* fix double free problem in encoder.c (#194)
  Thanks to @momo-trip

* Serucity fix for #200, heap buffer overflow in debug palette function.
  Thanks to @err2zero

* add EXTRA_DIST for LICENSE files (#129)
  Thanks to @ttdoda

* Travis-ci: added support for ppc64le (#140)
  Thanks to @dthadi3

* export sixel_allocator_new to dll (#151)
  Thanks to @johnnychen94

* README: Add Idris 2 language bindings (#155)
  Thanks to @Kaiepi

* performance: If width and height are unchanged, nothing to do. (#170)
  Thanks to @rokuyama

* README: add MacPorts to install options (#183)
  Thanks to @barracuda156

* fix for bash completion (#189)
  Thanks to @rcorre

* Add backport feature (nanosleep) for windows, github actions CI (#202)
  Thanks to @Kreijstal

* README: update NixOS link (#204)
  Thanks to @max-amb

* build: Remove override of $LIBJPEG_CFLAGS and $LIBJPEG_LIBS set by
  PKG_CHECK_MODULES()

* fix Problems with the dithering palette calculation (#188)
  Thanks to @gnachman, @j4james

* fix SEGV error in sixel_encoder_setopt (#174)
  Thanks to @shinibufa , @j4james

* curl: send original UserAgent header: "libsixel/${LIBSIXEL_VERSION}"

* fix heap-buffer-overflow in error_diffuse, quant.c:876 #172
  Thanks to @waugustus

* fix Heap-buffer-overflow in scale.c:214 #179
  Thanks to @chameleon10712, @j4james

* build: fallback support for environments without pkg-config.

* fix double-free problem in loader.c (#150)
  Thanks to @duytai, @ctrlcctrlv

* fix an assertion issue in stbi__create_png_image_raw (#163)
  Thanks to @kdsjZh, @dankamongmen

* Update stb_image.h from upstream to version 2.30
  THanks to @hzeller

* Update examples/drawing: add SGR-Pixels mode

* fix a problem on monochromatic encoded (-e) output (#112)
  Thanks to @interkosmos, @j4james

* fix a FPE issue (#166, #167)
  Thanks to @waugustus, @j4james

* cli: fix a scaling issue introduced in v1.6.1, which is caused
       when one of -w/-h is a percentage and the other is unset or "auto"

* fix a memory leak ploblem (#164)
  Thanks to @muetzenmann, @j4james

Author: tsutsui

graphics/libsixel/Makefile

@@ -1,7 +1,6 @@
-# $NetBSD: Makefile,v 1.77 2025/08/30 22:45:27 wiz Exp $
+# $NetBSD: Makefile,v 1.78 2025/08/31 09:30:45 tsutsui Exp $
 
-DISTNAME=		libsixel-1.8.6
-PKGREVISION=		28
+DISTNAME=		libsixel-1.8.7
 CATEGORIES=		graphics
 MASTER_SITES=		${MASTER_SITE_GITHUB:=saitoha/}
 GITHUB_PROJECT=		libsixel

graphics/libsixel/distinfo

@@ -1,5 +1,5 @@
-$NetBSD: distinfo,v 1.27 2021/10/26 10:46:29 nia Exp $
+$NetBSD: distinfo,v 1.28 2025/08/31 09:30:45 tsutsui Exp $
 
-BLAKE2s (libsixel-1.8.6.tar.gz) = 751303e6686b78b05cb6c96d14db406bf603964f00f685330190457f497e1053
-SHA512 (libsixel-1.8.6.tar.gz) = 460a5d46bada705b4a7ec2e250e2bde7f4b8ba155281fa5a102d1121f79c1c79145f042cd1f4bf93a2fc29e784e89afd539e963470663fc659d4fe72ea49fdaf
-Size (libsixel-1.8.6.tar.gz) = 4791628 bytes
+BLAKE2s (libsixel-1.8.7.tar.gz) = 30ff2df59714ffad327568a00f0e1244fa73b1cdd36e07fc2a723c4a657d56da
+SHA512 (libsixel-1.8.7.tar.gz) = 269e54f8c91f31e03498b8f6ae7b5968cf4236a5c0b68dd551227bbbca4fd47f91fa1bc162546b0f8b9a53dfa3d9b0ab872e9af37299f66d8259f0aab8f61050
+Size (libsixel-1.8.7.tar.gz) = 4888313 bytes