libvpx

Adaptive motion search control

This commit enables adaptive constraint on motion search range for
smaller partitions, given the motion vectors of collocated larger
partition as a candidate initial search point.

It makes speed 0 runtime of bus at CIF and 2000 kbps goes from
167s down to 162s (3% speed-up), at 0.01dB performance gains. In
the settings of speed 1, this makes the runtime goes from 33687 ms
to 32142 ms (4.5% speed-up), at 0.03dB performance gains.

Compression performance wise, it gains at speed 1:
derf  0.118%
yt    0.237%
hd    0.203%
stdhd 0.438%

Change-Id: Ic8b34c67810d9504a9579bef2825d3fa54b69454