vp9-svc: Allow for 2 stage downscaling for spatial layers.

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