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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

--- a/vp9/encoder/vp9_svc_layercontext.c

+++ b/vp9/encoder/vp9_svc_layercontext.c

@@ -661,6 +661,10 @@

   // averaging filter), use bilinear for now.

   if (width * height <= 640 * 480) {

     cpi->svc.downsample_filter_type[cpi->svc.spatial_layer_id] = BILINEAR;

+    // Use Eightap_smooth for low resolutions.

+    if (width * height <= 320 * 240)

+      cpi->svc.downsample_filter_type[cpi->svc.spatial_layer_id] =

+          EIGHTTAP_SMOOTH;

     cpi->svc.downsample_filter_phase[cpi->svc.spatial_layer_id] = 8;

   }