Refactoring: split prediction+extension for each plane, so we can
handle luma/chroma supertx pred in different ways.
Compatibility fix: fix conflicts with cb4x4 and chroma_sub8x8, now
for chroma sub8x8 supertx, only the top-left(basic cb4x4) or the
the bottom-right(cb4x4 + chroma_sub8x8) predictor will be used
without any blending within a 8x8 unit.

Change-Id: I6cf7b12768a82d3c7e01811ada02de84af9bd8ac