dav1d 0.5.2 'Asiatic Cheetah', the fast and small AV1 decoder

This is a minor update of the 0.5.x version of dav1d, the fast and small AV1 decoder,
codename 'Asiatic Cheetah'.
It supports all the AV1 features and all bitdepths.

0.5.2 brings improvements in speed for ARMv7 CPUs (a few % speedup), on
top of the already big speedups of 0.5.1. It should also improve the
speed for large videos on all platforms.

It also fixes minor issues, brings stability and introduces an OBU demuxer.