dav1d 0.5.0 'Asiatic Cheetah', the fast and small AV1 decoder

This is the fifth major release of dav1d, the fast and small AV1 decoder,
codename 'Asiatic Cheetah'.
It supports all the AV1 features and all bitdepths.

0.5.0 brings large improvements in speed on SSSE3 CPU (up to 40% speedup),
new speed improvements on AVX-2 (for 4-7%) and ARM64 (up to 10%) and ARM32.
It introduces some VSX, SSE2 and SSE4 optimizations.

0.5.0 fixes some minor issues, can export ITU T.35 metadata and improves
the player example.