Add AVS2/AVS3 codec support

There is a special build of VLC with support for these chinese codec floating around. https://github.com/xatabhk/vlc-3.0-avs2-avs3

It uses:

https://github.com/uavs3/uavs3d for decoding (BSD)
enable support in FFmpeg (the patch seems to be upstream)
patches VLC to add support for AVS2 decoder (GPL) via a davs2 library that seems to be this library.

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