EXT_INTER experiment

EXT_INTER experiment

NEW2MV is enabled, representing a new motion vector predicted from
NEARMV. It is mostly ported from nextgen, where it was named
NEW_INTER.
A few fixes are done for sub8x8 RDO to correct some misused
mv references in the original patch.
A 'bug-fix' for encoding complexity is done, reducing the additional
encoding time from 50% to 20%. In sub8x8 case, the old patch
did motion search for every interpolation filter (vp9 only
searches once). This fix also slightly improves the coding gain.
This experiment has been made compatible with REF_MV and EXT_REFS.

Coding gain (derflr/hevcmr/hevchd): 0.267%/0.542%/0.257%

Change-Id: I9a94c5f292e7454492a877f65072e8aedba087d4