This commit takes a preliminary attempt to refine the motion search
control. It detects the SAD associated with mv predictor per reference
frame, and based on which to determine whether the encoder wants to
reduce the motion search range (if the predicted mv provides fairly
small SAD), or to skip the current reference frame (if there exists
another ref frame that gives much smaller SAD cost).

This feature is turned on in the settings of speed 1 and above.

In speed 1, compression performance changed
derf  -0.018%
yt    -0.043%
hd    -0.045%
stdhd -0.281%

speed-up
pedestrian_area_1080p at 4000 kbps 100 frames
199651ms -> 188846ms (5.5% speed-up)
blue_sky_1080p at 6000 kbps
443531ms -> 415239ms (6.3% speed-up)

In speed 2, compression performance changed
derf  -0.026%
yt    -0.090%
hd    -0.055%
stdhd -0.210%

speed-up
pedstrian 113949ms -> 108855ms (4.5% speed-up)
blue_sky  271057ms -> 257322ms (5% speed-up)

Change-Id: I1b74ea28278c94fea329d971d706d573983d810d