Fixes to inter decoding and some initial sub8x8 support.
lavc/vp9: add inter pred itxfm_add loops.

WIP: untested except compilation.
Add minor FIXME.
Add emulated edge support.
Add one more minor FIXME.
Fix loopfilter mode/ref level deltas.
Skip inner loopfilter edges for inter blocks w/o coefficients.
Three bugfixes for almost complete inter frame support.

First, unset the high-precision bit if the reference MV is too large
to have a high-precision bit anyway. Secondly, skip inner loopfilter
edges for the 4x4 transform also. Lastly, don't clip reference MVs in
sub8x8 blocks.
Fix a few more inter decoding bugs.

Fixes decoding of first ARF and subsequent non-ARF (Y plane only)
in vp90-2-2pass-akiyo.webm. The frame after still has some artifacts.
Fix backward adaptivity tree mix-up.

Fixes another 2-3 frames (Y plane only) in vp90-2-2pass-akiyo.webm.
Fix bw adaptivity for inter frame coefficients.
Fix in find_ref_mvs() related to sign bias of second reference frame.
Another minor bug in find_ref_mvs().

First 20 frames in vp90-2-2pass-akiyo.webm (Y plane) decode correctly
now.
Workaround for bug in libvpx counting hp bits even if it's not coded.

If the reference MV is larger than 8 pixels (64 in subpixel q3), the
hp bit isn't coded for this particular block. However, we count it for
bw entropy purposes anyway. This will cause the bw entropy to be biased
towards the 1 bit being set.
Add minor FIXME and don't do bw adaptivity if hp bit is disabled globally.
Remove stray debug line.
Reference motion vector clamping.
Properly use second reference in 4x4 compound inter pred.

Fixes Y plane reconstruction on bus @ 1500kbps (1KF per 125 frames).
Fix possible overread in marker bit parsing.

Also remove a FIXME that was already fixed earlier.
Add copyright of other-than-me developer.