Changed to use reference mv as nearest mv

The reference motion vector selected by surrounding pixels that has
the best matching score is used as nearest motion vector.

The change has shown consistent gain on all test sets, compression
gains range from .2% to .6%. The variation is largely dependent on
various other experiments on or off.

Change-Id: I5552e1c2f6fc57c3e8818a5ee41ffda89af05e75