This commit builds the actual prediction block in sub-pixel accuracy
and uses which to calculate SAD for non-RD mode decision. In the trail
run on pedestrian_area at 1080p, rtc speed -7 runtime goes from
23495 ms -> 25107 ms (7% slower). The compression performance is
improved by 20.57% for rtc test set.

Change-Id: I438589cd103fe99f1b50c2d1939ac6ca43fa0157