vp9-svc: Use eightap_smooth for downsampling at low resol.

vp9-svc: Use eightap_smooth for downsampling at low resol.

Switch from bilinear to eighttap_smooth for frame-level
downsampling at low resolutions (<= 320x240).

avgPSNR/SSIM metrics increase from ~0.5-2% (all clips positive gain),
for 2 and 3 spatial layer SVC, with 3 temporal layers.
Small/negligible increase in encoding time (< 1%).

Change-Id: I758472fc4fddd51d87f13c9d1a1cd4986ef5d41f