Log Message:
graphics/gifski: update to 1.9.0

 - gifski can now read other GIF animations as an input. It's a very bad idea
   to try to use it as an optimizer, since it will inherit a bad quality and
   can only make it worse. However, combined with the --width option it can be
   useful for generating thumbnails of GIF animations.

 - it now uses multiple threads for resizing and preprocessing when reading
   videos, so might be faster on multi-core machines, especially when
   using --fast