a new way of determining reference motion vector

Using surrounding reconstructed pixels from left and above to select
best matching mv to use as reference motion vector for mv encoding.

Test results:
       AVGPSNR  GLBPSNR VPXSSIM
Derf:  1.107%   1.062%  0.992%
Std-hd:1.209%   1.176%  1.029%

Change-Id: I8f10e09ee6538c05df2fb9f069abcaf1edb3fca6
13 files changed