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.
Some minor fixes to get intra prediction hooked up.
Quick hack to read a new "marker" bit.

Now, the first few partition tree symbols in the first block of a
keyframe are decoded correctly. Still doesn't look like much.
More fixes. in intra mode reading.

The first intra mode of the first block of the first keyframe is now
read correctly. Coefficient decoding is likely still broken, although
the probabilities in the header are now read correctly. Reconstruction
itself is obviously still broken.
Fix coefficient decoding for first block of first keyframe.

Reconstruction still not correct, but coefficients and mode reading
is now correct.
Fix reconstruction bug.

First block (Y only) reconstructs correctly now. Some intra prediction
modes are still wrong, UV looks wrong (?) and overall reconstruction is
thus essentially mostly garbage.
Fix tx type h/v switchup.

First 8x8 block of vp90-02-size-64x64.webm decodes correctly now.
Fix bug in adst8_1d.

Now the first two 8x8 blocks decode correctly.
Fix bug in mode assignment and intra mode border availability check.

First 3 8x8 blocks of vp90-02-size-64x64.webm decode and reconstruct
correctly now. The fourth block has a correct Y/V plane, but U has a
slight DC problem.
Fix bug in UV intra mode border availability check.

Top 48x16 pixels of vp90-02-size-64x64.webm decode correctly now.
Fix ordering bug in idct8_1d.

Top 64x24 pixels in first keyframe of vp90-02-size-64x64.webm now
decode and reconstruct correctly.
Fix bugs in dc_top/dc_left 8x8/16x16 intra prediction.

Top half of vp90-02-size-64x64.webm decodes correctly now.
Fix sign inversion for 16-point 1d adst.

Fixes another reconstruction bug. The top left 3 48 pixel columns of the
3rd set of 16 pixel rows now decodes correctly. The rightmost 8x8 pixel
block in row 32-40 still has a minor artifact.
Fix bug in hor_up 8x8/16x16 intra prediction.

Top 64x48 pixels of vp90-02-size-64x64.webm decode correctly now.
Clear coefficient context if a skip-block occurs.

This fixes the remainder of the keyframe reconstruction bugs in
vp90-02-size-64x64.webm. The loopfilter is still unimplemented.
Remove debug code.
Fix another bug in 8x8/16x16 hor_up intra prediction.
Fix offsets for destination pointer between 64x64 SBs.

Appears to fix reconstruction of first keyframe in vp90-02-size-66x66.webm.
Add 32x32 DC/TM/H/V intra pred functions.

Also add placeholders for the diagonal intra prediction ones.
decode_uniform() should read ceil(log2(x)) bits, not log2(x).