VP9 decoder.

Currently does complete header parsing and parts of keyframe block
parsing. Most notably, reconstruction itself is not yet completed
(it only does 4x4/8x8/16x16 intra prediction, not 32x32, and it
also only does 4x4/8x8/16x16 inverse transforms (both adst as well
as dct) and lossless 4x4 iwht, but not yet 32x32 (idct)); in addition,
loopfilter, inter frame handling and backward adaptivity are missing.

Lastly, and most importantly, there are not yet any tests, and the
code itself is untested, i.e. there's likely typos, silly bugs and
other bad behaviour in this code.