Improves subpixel reference mv evaluation

Improves subpixel reference mv evaluation

Previously, in evaluating reference motion vectors, MVs are always
rounded to integer pixel position and SADs are calculated.  This
commit takes into account the subpixel portion of the mvs, and uses
bilinear interpolation to produce reference pixel values in subpixel
postions. In addition, SSE is used in place of SAD. Pixels used are
16x2 above and 2x16 to the left.

This commmit intends to test the potential of this line of work in
term of compression improvement, obviously, the change would increase
decoder complexity significantly.

Test results
std-hd: 1.738%(avg) 1.779%(glb), 1.663%(ssim)
derf: 0.472%(avg) 0.477%(glb), 0.418%(ssim)

Change-Id: I3ae1b098f6289df78891134d9a5e4bb2fde87a0b