Combinations of different mv modes for two reference frames
are allowed in compound inter modes. 9 options are enabled,
including NEAREST_NEARESTMV, NEAREST_NEARMV, NEAR_NEARESTMV,
NEAREST_NEWMV, NEW_NEARESTMV, NEAR_NEWMV, NEW_NEARMV, ZERO_ZEROMV,
and NEW_NEWMV.
This experiment is mostly deported from the nextgen branch.
It is made compatible with other experiments

Coding gain of EXT_INTER(derflr/hevcmr/hevchd): 0.533%/0.728%/0.639%

Change-Id: Id47e97284e6481b186870afbad33204b7a33dbb0