dav1d 0.4.0 'Cheetah', the fast and small AV1 decoder

This is the fourth major release of dav1d, the fast and small AV1 decoder, codename 'Cheetah'.

It supports all the AV1 features and all bitdepths.

0.4.0 brings large improvements in speed on ARM64 (up to 25% speedup) and minor improvements on SSE and ARM.

It also improves the RAM usage quite significantly, sometimes more than halving the RAM used.