dav1d 0.8.1 'Eurasian hobby' the fast and lean AV1 decoder

This is a minor update of the dav1d decoder, from the 0.8.x branch.

This releases fixes a regression in the buffer reference handling
introduced by the buffer pool added in 0.8.0. dav1d_close() invalidated
all buffer references.
It also contains further ARM32 high bit depth SIMD optimizations, x86 wiener
filter optimizations and fixes a couple of minor issues.