Support FFMPEG 5

trunk rev 1b3acc85

My distro archlinux just upgraded from ffmpeg 4.4 to ffmpeg 5.0 . There is a ffmpeg4.4 legacy package for vlc and other pacakges that don't support ffmpeg 5 (yet) .

openmw trunk builds that were built against ffmpeg4.4 continue to work with the legacy pacakge present, but new builds against ffmpeg 5 get stuck at runtime.

Starting openmw goes as normal, but once on the new/load/save/options/credits/exit screen things change. You can select the options but the screen doesn't change and openmw is somehow stuck. music keeps playing, mouse keyboard and active programs keep running and respond .

Openmw doesn't crash , but is unplayable . It does respond cleanly to a kill -9 command from terminal .

For now i've put ffmpeg, vlc and x264 on my ignore updates list as a temporary stopgap .

Should I look into building openmw against the legacy ffmpeg pacakge (may need a patch to convince cmake to use ffmpeg4.4 ) or are there plans to support ffmpeg 5 in openmw ?