bug#39703: MPV chooses the slow AV1 decoder

[Leo Famulari]

I noticed that our MPV cannot play back AV1 videos in real-time on my
Thinkpad x230 laptop.

MPV gets AV1 playback support from FFmpeg. Our FFmpeg package includes
both the libaom and dav1d AV1 decoders. I think that MPV is using
libaom, and that is too slow.

I rebuilt FFmpeg without libaom, and MPV can play my AV1 test file fine
now, using dav1d.

I wonder, is there a way to change the preference?