dav1d 0.8.0 'Eurasian hobby' the fast and lean AV1 decoder

This is a major update of the dav1d, the fast and lean AV1 decoder,
codename 'Eurasian hobby'.

This release improves, once again, the speed on all platforms.

On the desktop platforms, it improves the performance (up to 10%)
by doing less allocations and adding more optimizations for AVX2 and SSE.

On the ARM platforms, this release brings numerous improvements for ARM32
and notably optimizes most of the 10/12bit bitdepth filters.