vp9: SVC feature to use partition from lower resolution.

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