Add support for webp animation in multi page file

[Sprintdriver]

Hi.

Webp files does support animation. As a workaround today, I can make several images into a single multiple pages (or frames) webp file using Gimp with layers.

The nice thing about webp files with multiple pages is the compression, which I figured is a good feature when having a lot of less-important but still-need-to-keep image files.