[FFmpeg-devel] Animated WebP decoder

[Martin Reboredo]

v3 Update:
- Add a libwebpdecoder based WebP decoder for animated pictures.
- Adapt the native decoder for the new demuxer.

v2 Update:
- Cleanups based on reviews.

This patchset introduces an initial implementation of a decoder for the animated WebP format. The native decoder doesn't provide support for it and this implementation
decodes frame by frame instead of proceeding with the entire file instead. Otherwise, in the case of a sole image it will send the data completely.