vp9: SVC feature to use partition from lower resolution.
For SVC with 3 spatial layers: Add feature to copy/upscale partition from middle spatial layer to the upper/highest resolution, when superblock sad is not high. Enabled for speed >= 7 and only for non-reference frames. Speedup ~3-4%, small loss in avgPNSR/SSIM of ~1%. Change-Id: I7f0a2716c0fde28bade0f86159d11b7e31d6ab8d
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