Remove a VLA.
intra-pred: re-roll 8x8 16x16 and add 32x32.
Inter frame reconstruction.

Currently does >= 8x8 block prediction for the Y plane. Sub8x8 DSP code
exists, but the wrapper code in vp9.c is missing. Likewise, UV plane
handling in vp9.c is missing. Lastly, inverse transform is missing.
Slight simplification in bilinear filter calculation.
Remove a cosmetically unpleasing space after unary '-' operator.
Backward adaptivity for inter-frame probabilities.
More dynamic 16px loopfilter functions.

These can handle variable loopfilter levels or loopfilter widths, thus
allowing for more 16px-per-call SIMD implementations. The same concept
can in the future be used to create 32px-per-call functions once AVX2
becomes real.
Remove some excessive inlining in the MC functions.

Reduces binary size of vp9dsp.o from 527kb to 416kb (for comparison:
vp8dsp.o is 120kb; without MC, vp9dsp.o is 225kb).
Fix a minor FIXME in check_intra_mode(), and remove an obsolete FIXME.
lavc/vp9: use init_get_bits8().
lavc/vp9: use decode012().