When the neighbor uses a sub8x8 partition, to predict chroma pixels in
the overlapping region, instead of averaging all mvs in the 8x8 block,
only apply those belonging to neighboring sub8x8 parts to the
corresponding sub-regions (no averaging).
The change applies to both settings w/ and w/o SUB8X8_MC experiment.

Change-Id: I033e9f218d02bc9879841b794931b575fac14f25