vp9-svc: Allow for 2 stage downscaling for spatial layers.
For 1 pass cbr mode: allow for two-stage 1:2 scaling (which will use the 1:2 optimized scaler) if the spatial layer is 1/4x1/4 of souce. Without this change, the base layer for 3 spatial layers would be using the non-normative scaler which is un-optimized/C code. Change-Id: I9d73f92a4a96927d0f1d6bf75315c1e60513226a
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