[3.0] avcodec: fix build with FFmpeg 8
3 backports and one for FF_LEVEL_UNKNOWN which wasn't used by VLC 4 anymore.
Fixes #29278
Edited by Steve Lhomme
3 backports and one for FF_LEVEL_UNKNOWN which wasn't used by VLC 4 anymore.
Fixes #29278
VideoLAN code repository instance