Fix undefined behavior in x264_ratecontrol_mb
Fix array overread in quant matrix initialization
Caused miscompilations in gcc 4.8.
x86inc: create xm# and ym#, analagous to m#

For when we want to mix simd sizes within one function.
x86: more AVX2 framework, AVX2 quant_8x8

Use 32-byte alignment instead of 16 to satisfy AVX2 using new macros.
Change the alignment of malloc.
avx2 idct 16x16
test
(not working)