Log Message:
libsixel: update to 1.8.1.

No upstream changelog in the NEWS file, but includes the following fixes:

* Fix critical bug: sixel_dither_set_diffusion_type()
  (called from img2sixel -d option) doesn't work well
* Suppress an uninitialized warning on GCC-4.1
* Move pragma GCC diagnostic to outside functions because
  it's not allowed on gcc44