libsixel-1.8 released

              ------------------------------
               What's new in libsixel-1.8 ?
              ------------------------------

• core: Upgrade stb_image to 2.19.

• core: Introduce new dithering method, a_dither / x_dither (http://pippin.gimp.org/a_dither/).
  Thanks to @hodefoting.
  #53

• core: Fix wrong HLS color handling.
  0fb35d2

• core: Improve quality of 15bpp(hi-color mode) dither.
  42f3428

• img2sixel: Allow a deferred clear code in a GIF format
  GIF decoder must do nothing when the table is full.
  See Section "DEFERRED CLEAR CODE IN LZW COMPRESSION" in
  https://www.w3.org/Graphics/GIF/spec-gif89a.txt.
  Thanks to @mame.
  #63

• img2sixel: Marks -D option (read source images from stdin continuously) as deprecated
  9c8ffa6

• Some bug fixes and minor improvements.
  Thanks to @set135, @ttdoda.

• Announcement for package maintainers:
  Immutable tarball is provided, because GitHub auto-generated tarball may be not immutable.

  https://github.com/saitoha/libsixel/releases/download/v1.8.0/libsixel-1.8.0.tar.gz

  For details, see #64 .