dav1d 0.6.0 'Gyrfalcon', the fast and lean AV1 decoder

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

0.6.0 brings major improvements in 10/12bit decoding on ARMv8 CPUs,
up to 2.5 times faster than 0.5.2.
It also brings new AVX-512, AVX2 and SSSE3 optimizations and improves
the existing optimizations on all platforms.
Finally, it also fixes some decoder mismatches and minor crashes.