This commit estimates the motion vector rate cost right after full
pixel motion search. It combines this and the mode cost and compares
the corresponding rate-distortion cost. If it is already above the
current best one, skip the rest sub-pixel motion search and modeling
process. For pedestrian_area 1080p at 4000 kpbs, the speed -5 runtime
goes down from 39425 ms -> 38399 ms.

Change-Id: If4cd7119fd6c266798d5cf1d19d19ab425e52a26