[FFmpeg-devel] Add support for animated WebP

[Thilo Borgmann]

Hi,

support for animated WebP had been proposed twice in the past [1][2].
This did stall when Lynne called for some review on the demuxer in [1].

So these are the rebased patches from [1], pinging especially for demuxer review.
Retested decoding with all files given in #4907.
Demuxer needs testing, I just rebased it.

-Thilo

[1] https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2020-September/269584.html
[2] https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2021-September/285204.html