motion: remove NULL checks prior to X265_FREE calls
motion: use new pixelavg_pp primitive for lowres QPEL pixel generation
use x264 pixel average assembly routines for bidir and lowres QPEL

This required adding a weight parameter and re-ordering arguments.  Bidir might
eventually use the weighting feature so this didn't seem like a bad trade-off.

This commit naively pulls in all of mc-a.asm from x264 for just this one
set of assembly functions.
fix bug on SAO initialize
pixel: use unaligned loads for reference pixels in sad_24
pixel: fix typo which was copied and pasted 36 times