Enable rectangular support for comp inter-intra

This commit enables rectangular block prediction of compound
inter-intra mode. It combines the mb/sb32/sb64 prediction functions
into a unified version with configurable block width and height.
This fixes the enc/dec mismatch of the codebase when
comp-interintra-pred is enabled.

Change-Id: I1d0db2f1f184007802df04fcd12b9dadb3189ff0