[VLC3] Support FFmpeg 5.0 (except for VAAPI)

After reading #26498 (closed), I wanted to investigate how much work would be necessary to support FFmpeg 5 on VLC3.

In this WiP MR, I applied simple changes manually, and cherry-picked some commits from VLC4 to fix some others.

In the end, it seems there is only one issue remaining: FFmpeg removed the VAAPI context/header, so there is no struct vaapi_context anymore. This has been fixed by 08cbeca2 on VLC4, but the patch does not apply on VLC3 and need rework (there is no vaapi_dec_pic_context for example).

If this can be adapted and all the changes are acceptable on VLC3, I think it would be nice to be able to build VLC3 with FFmpeg 5 (it would simplify everyone's life).