vp9-svc: Set downsampling filter for VGA layer.
vp9-svc: Set downsampling filter for VGA layer.

Downsampling filter for SVC was set to subsample (phase 0)
for HD -> VGA, and bilinear averaging (phase 8) for VGA -> QVGA.
This change makes it bilinear averaging for HD -> VGA.

Given the recent commit 9f9d4f8, quality is improved with
this change: avgPSNR/SSIM up ~1-3% on HD clips in RTC set.
Speed decrease of ~1% for 3 layer SVC.

Change-Id: If834a320e372b8b922a6bf7cab4227703b1beae6

webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx