Overlapped block motion compensation experiment

Overlapped block motion compensation experiment

In this experiment, an obmc inter prediction mode is enabled for
>= 8X8 inter blocks. When the obmc flag is on, the regular block-
based motion compensation will be refined by using predictors of
the above and left blocks.
Fixed some compatibility issues with vp9_highbitdepth, supertx,
ref_mv, and ext_interp.

Coding gain (%) on derflr/hevcmr/hevchd
OBMC:
1.047/1.022/0.708
OBMC + SUPERTX:
1.652/1.616/1.137
SUPERTX:
0.862/0.779/0.630

Change-Id: I5d8d3c4729c6d3ccb03ec7034563107893103b7f