This commit allows the sub8x8 blocks to compose and filter their
chroma components for supertx in cb4x4 mode. The coding gains of
supertx and cb4x4 are largely additive:

          supertx      cb4x4       cb4x4 + supertx
lowres     -1.0%       -2.7%        -3.64%
midres     -0.8%       -1.3%        -2.10%

Change-Id: Ie7d09f6fceb36ce375e56773728f05dd628786fe