Adapt FFmpeg 5 API changes

• 3 commits related to API constification. Relevant changes in FFmpeg:
  • ffmpeg/56450a0ee4fdda160f4039fc2ae33edfd27765c9 (AVInputFormat and AVOutputFormat)
  • ffmpeg/626535f6a169e2d821b969e0ea77125ba7482113 (AVCodec)
• 1 commit changing int to size_t. Relevant changes in FFmpeg:
  • ffmpeg/14040a1d913794d9a3fd6406a6d8c2f0e37e0062 (announcement)
  • ffmpeg/ef6a9e5e311f09fa8032974fa4d0c1e166a959bb (change)