Currently only supports single image frames (no animation possible).
If quality slider is set to 100 then lossless compression will be used,
otherwise lossy compression is used.
Gives about 35% reduction of filesize save when re-saving splash screens with lossless
compression.
Also saves much faster, up to 15x faster than PNG with a better compression ratio as a plus.
Note, this is currently left disabled until we have WebP libs (see T95206)
For testing precompiled libs can be downloaded from Google:
https://storage.googleapis.com/downloads.webmproject.org/releases/webp/index.html
Differential Revision: https://developer.blender.org/D1598
