x86: SSE2 and SSSE3 plane_copy_deinterleave_rgb

About 5.6x faster than C on Haswell.
x86inc: free up variable name "n" in global namespace
Fix corruption with CAVLC overflow handling in MBAFF+main profile

Probably a regression in 83561e.
configure: correctly support gpac 0.5.0
Macroblock tree overhaul/optimization

Move the second core part of macroblock tree into an assembly function;
SIMD-optimize roughly half of it (for x86). Roughly ~25-65% faster mbtree,
depending on content.

Slightly change how mbtree handles the tradeoff between range and precision
for propagation.

Overall a slight (but mostly negligible) effect on SSIM and ~2% faster.