Add idct32_1d
Fix intra prediction bug in 16x16 hor_up.
Remove trailing whitespaces.
Initial implementation of loopfilter.

Fixes decoding of the first frame (post-lf) of vp90-02-size-64x64.webm.
Allow 16-pixel loopfilter functions.

We could (future) allow 32-pixel (for AVX2) also, but that's not
necessary ATM. It'd also be nice to allow to mix 4-wd and 8-wd
functions in a single call; this would give speed-ups in SIMD
versions, although the C might be slightly slower. For now, I'll
leave that as a future TODO.
Backup pre-loopfilter data for next sb-row intra prediction.
Fix loopfilter ordering bug, and turn loopfilter off if level=0.

Fixes loopfilter-related mismatches in e.g. vp90-2-00-quantizer-01.webm.
Fix bug where row-edge loopfilter was applied to the top lines.

Fixes some artifacts on the top of first keyframe in
vp90-2-00-quantizer-55.webm.
Clip filtermask to signed 8bit.

Fixes another few mismatches on vp90-2-00-quantizer-55.webm.
Another clipping bug in loopfilter.

Fixes another few mismatches in vp90-2-00-quantizer-55.webm.
Add basic reference frame handling.

This allows inter frames to proceed to actual block parsing, where it
fails because inter frame handling is completely missing.
diag_downleft: re-roll 8x8 16x16 and add 32x32.
diag_downright: re-roll 8x8 16x16 and add 32x32.