Keep track of the best and second best full pixel motion vector
candidates, and do subpel search around both of them.

Compression improvement:
lowres 0.22%   midres 0.23%   hdres 0.18%

No noticeable encoding speed changes observed on lowres test clips.

Change-Id: I5f4df2a03d1db061cfdfdba6138b27e9ea91f089