This commit makes the sub8x8 chroma component inter predictor
operate at 2x2 block level. This allows one to use the actual motion
vector associated with each individal pixel block. It improves the
compression performance

lowres  0.40%
midres  0.25%
hdres   0.15%

Change-Id: Ia40e07cc7fde463dbf660018850e024932136c4f