Adds support for a gifski_pass in video_formats and a gifski format
which utilizes it. Resolves#79
Much consideration was put into how best to implement this. Gifski does
not support piped input, and implementing such functionality is
infeasible as the ffmpeg library used by Gifski requires inputs be
physical files. Consequentially, an intermediate file of some sort is
required. While an implementation that would not require a prior ffmpeg
pass was investigated, the only viable intermediary format
between Pillow and Gifski's ffmpeg decoder is an animated png and gifski
will never pass this png to it's ffmpeg decoder as it sees the png
header and instead interprets the file as a single frame image.
Austin Mroz
a2900c3273