vp9-svc: Allow for nonzero motion on spatial reference.

vp9-svc: Allow for nonzero motion on spatial reference.

For nonrd pickmode on a given spatial layer, the spatial
(golden) reference was always only using zeromv for prediction.
In this patch if the downsampling filter used for generating
the lower spatial layer is an averaging filter (nonzero phase),
we allow for subpel motion on the spatial (golden) reference to
compensate for the shift. This is done by forcing the testing of
nonzero motion mode to compensate for spatial downsampling shift.

Improvement for cases where the downsampling is averaging filter.
In the current code this is only done for generating
resolutions <= QVGA.

Improvement for avgPSNR/SSIM on RTC set for speed 7: ~1.2%.
Gain is larger (~2-3%) for VGA clips with 2 spatial layers.
~1% speed slowdown for 3 layer SVC on mac.

Change-Id: I9ec4fa20a38947934fc650594596c25280c3b289