vp9: just disallow temporal or no-update segmentation on size-change.

The spec doesn't describe how it should be decoded so this is probably
the safest thing to do. Fixes valgrind errors on fuzzed11.ivf and fixes
valgrind errors on fuzzed10.ivf differently.
vp9: coef context reading optimization.
vp9: faster skip-block coefficient blanking.
vp9: more aligned zeroes.
vp9: decode_mode context optimizations.
vp9: coef decoding context-splat speedup.
vp9: remove memset in loopfilter level setting loop.

Use same function for segmentation map writing also.
vp9: invert order in l[] intra prediction array.

The directional intra predictors either don't care about order (dc, h,
dc_left, tm), or they prefer inverted order (vr, dr, hd). This allows
more efficient SIMD implementations.
vp9: fix compilation with !HAVE_FAST_64BIT.

Spotted-by: Carl Eugen Hoyos <cehoyos@ag.or.at>