[bug#39714] Real-time AV1 playback in VLC

[Leo Famulari]

On my Thinkpad x230, VLC cannot play back AV1 videos in real-time. They
stutter and drop frames.

This is because our VLC's AV1 playback is being provided by FFmpeg,
which by default chooses libaom, which is too slow.

By building VLC with the dav1d AV1 decoder, VLC chooses to use dav1d for
playback, and everything is fine.

Here are some AV1 samples; I like the 1080 "Summer Nature" clip:

https://www.elecard.com/videos