Downsample SAD computation in motion search

Downsample SAD computation in motion search

Added a speed feature to skip every other row
in SAD computation during motion search.

                 Instruction Count        BD-Rate Loss(%)
cpu   Resolution   Reduction(%)    avg.psnr   ovr.psnr    ssim
 0       LOWRES2      0.958         0.0204     0.0095    0.0275
 0       MIDRES2      1.891        -0.0636     0.0032    0.0247
 0        HDRES2      2.869         0.0434     0.0345    0.0686
 0       Average      1.905         0.0000     0.0157    0.0403

STATS_CHANGED

Change-Id: I1a8692757ed0cbcb2259729b3ecfb0436cdf49ce