Use diamond search to replace full search in full-pixel refining search

In NEWMV mode, currently, full search is used as the refining search
after n-step search. By replacing it with an iterative diamond search
of radius 1 largely reduced the computation complexity, but still
maintained the same encoding quality since the refining search is
done for every macroblock instead of only a small precentage of
macroblocks while using full search.

Tests on the test set showed a 3.4% encoding speed increase with none
psnr & ssim loss.

Change-Id: Ife907d7eb9544d15c34f17dc6e4cfd97cb743d41