arm: implement x264_pixel_var_8x16_neon

checkasm --bench on a cortex-a9:
var_8x16_c: 4306
var_8x16_neon: 791
arm: implement x264_pixel_var2_8x16_neon

checkasm --bench on a cortex-a9:
var2_8x16_c: 5677
var2_8x16_neon: 1421
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.