libvpx

Enable real-time version reference motion vector search

This commit enables a fast reference motion vector search scheme.
It checks the nearest top and left neighboring blocks to decide the
most probable predicted motion vector. If it finds the two have
the same motion vectors, it then skip finding exterior range for
the second most probable motion vector, and correspondingly skips
the check for NEARMV.

The runtime of speed -5 goes down
pedestrian at 1080p 29377 ms -> 27783 ms
vidyo at 720p       11830 ms -> 10990 ms
i.e., 6%-8% speed-up.

For rtc set, the compression performance
goes down by about -1.3% for both speed -5 and -6.

Change-Id: I2a7794fa99734f739f8b30519ad4dfd511ab91a5