Aligned clears to cover out of visible image edges.
lavc/vp9: use init_get_bits8().
lavc/vp9: use decode012().
Initial fate tests.

Keyframe-only for now (i.e. -frames:v 1), because keyframes are
not yet complete so they would just produce noise. References
were produced using vpxdec $$ -o test.y4m; ffmpeg -i test.y4m
-frames:v 1 -f framemd5 tests/ref/fate/vp9-$$.

Currently, the following tests fail for a variety of FIXMEs:
fate-vp9-00-quantizer-00.webm
fate-vp9-02-size-NxM.webm for several N/M combinations
fate-vp9-03-size-NxM.webm for several N/M combinations
fate-vp9-emu-edge-* (several additional ones beyond non-emuedge)
Add TODO list so we can track outstanding items easier.
Fix lossless (fate-vp9-00-quantizer-00) support.
Workaround loopfilter bug (in libvpx).

for UV loopfilter, skip odd row loopfilter if odd col loopfilter of
that same 8x8 pixel area was skipped also. This triggers, for example,
on the right side of vp90-2-02-size-66x66.webm; because the number of
cols is a not a multiple of 2, we skip the odd col loopfilter around
pixels at position x=68. However, because of a bug in libvpx, we have
to skip the row loopfilter around pixels at y=4, y=12, y=20 and y=28
(i.e. all odd rows) between x=64-68 also.

This fixes vp90-2-02-size-66x66.webm keyframe reconstruction to be
identical to what libvpx produces. Of vp90-2-02-*.webm, all files
except vp90-2-02-size-18x66.webm produce identical keyframe output
to libvpx now. vp90-2-03-size-*.webm still have several bugs.
Extend edges for intra prediction.

Fixes keyframe reconstruction in fate-vp9-02-size-18x66.
Skip reconstruction and coefficient reading for fully invisible blocks.

This fixes keyframe reconstruction of all remaining non-emuedge tests
except fate-vp9-03-size-226x196.
Clean out-of-visible frame coefficient contexts.

See set_contexts_on_border() in libvpx, this fixes keyframe
reconstruction for fate-vp9-03-size-226x196.
Fix buffer overwrite in hor_up intra prediction function.

Fixes keyframe reconstruction on e.g. fate-vp9-emu-edge-00-quantizer-29.
Don't touch 8wd/16wd loopfilter source data in 4wd implementation.

Fixes segfaults in remaining fate-vp9-emu-edge-* tests - all tests now
pass succesfully in keyframe decoding.
Minor cosmetic change.
Align a_buf (for later SIMD purposes).
Save eob for each decoded coefficient block.

Use this to skip itxfm_add() if eob==0. In the future, we can use this
to optimize itxfm_add for low eob values (e.g. DC-only, or low-order
subblock for larger transforms.
Add missing const.
Transpose inverse transform coefficients during reading.

In the future, this allows for faster SIMD, since it only needs one
transpose (between the two 1D inverse transforms) instead of two (one
before each 1D inverse transform).
lavc/vp9: use designated initializers in mv_ref_blk_off.
lavc/vp9: fix sign of mv_ref_blk_off values.