Fix multi-thread encoding for cb4x4 mode
Fix multi-thread encoding for cb4x4 mode

This commit makes the encoder to properly account for all transform
block sizes when combining statistics from encoding threads.

Change-Id: I010acd3b247dc890f63756d3d1436b1fb52ea2d9

GrokImageCompression/aom,smarter/aom,luctrudeau/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,luctrudeau/aom,GrokImageCompression/aom,luctrudeau/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom