vp9 svc: SVC encoder speed up.
Bias towards base_mv and skip 1/4 pixel motion search when using base mv. 2~3% speed up for 2 spatial layers, 3~5% speed up for 3 spatial layers. PSNR loss: (2 layers) 0.07dB for gips_stationary, 0.04dB for gips_motion; (3 layers) 0.07dB for gips_stationary, 0.06dB for gips_motion. Change-Id: I773acbda080c301cabe8cd259f842bcc5b8bc999
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