libsixel: update to 1.8.0.

Upstream changes (from NEWS):

                  ------------------------------
                  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.
        https://github.com/saitoha/libsixel/pull/53

* core: Fix wrong HLS color handling.
        https://github.com/saitoha/libsixel/commit/0fb35d22e7ba4ed8bce3be42791e6b39c3187b89

* core: Improve quality of 15bpp(hi-color mode) dither.
        https://github.com/saitoha/libsixel/commit/42f34283defea67c2d6294af4bd7dcb4fdf9f046

* 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.
            https://github.com/saitoha/libsixel/pull/63

* img2sixel: Marks -D option (read source images from stdin continuously) as deprecated
            https://github.com/saitoha/libsixel/commit/9c8ffa62203899d866fc28e3ab7c6dfd9a0019b1

* 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 https://github.com/saitoha/libsixel/issues/64 .

(tsutsui)