AV1 multithreaded playback doesn't work without audio

[bvibber]

With no audio, or audio muted, the multithreaded AV1 decoder doesn't play -- it shows an initial frame, then nothing more is returned.

This is probably due to the old video-without-audio control path which also handles other things poorly like late frames, combined with dav1d's pipelining.

Repro by muting before playback.