Weighted single motion search is implemented for obmc predictor.
When NEWMV mode is used, to determine the MV for the current block,
we run weighted motion search to compare the weighted prediction
with (source - weighted prediction using neighbors' MVs), in which
the distortion is the actual prediction error of obmc prediction.

Coding gain: 0.404/0.425/0.366 for lowres/midres/hdres
Speed impact: +14% encoding time
              (obmc w/o mv search 13%-> obmc w/ mv search 27%)

Change-Id: Id7ad3fc6ba295b23d9c53c8a16a4ac1677ad835c