vp9-svc: Enbale scale partition reference frames.

For reference frames: enable scale partition for
superblocks with low source sad or if bsize on lower-resoln
is at least 32x32.

Keep feature disabled for base temporal layer.

Small regression in avgPNSR/SSIM metrics, ~0.5-1%.
Speedup ~2-3% on mac for SVC (3 spatial/3 temporal layers) at speed 7.

Change-Id: I5987eb7763845b680059128b538bb5188be0cca5
2 files changed