Compound inter-intra experiment

A patch on compound inter-intra prediction.

In compound inter-intra prediction, a new predictor for
16x16 inter coded MBs are obtained by combining a single
inter predictor with a 16x16 intra predictor, in a manner
that the weight varies with distance from the top/left
boundary. The current search strategy is to combine the best
inter mode with the best intra mode obtained independently.

Results so far:

derf +0.31%
yt +0.32%
std-hd +0.35%
hd +0.42%

It is conceivable that the results would improve somewhat
with a more thorough search strategy where all intra modes
are searched given the best mv, or even a joint search for
the best mv and the best intra mode.

Change-Id: I7951f1ed0d6eb31ca32ac24d120f1585bcd8d79b