vp9: SVC: Add option to set downsampling filter type.
Add option in SVC to set the filter type and phase for the frame level downsampling filters. For 3 spatial layers: set downsampling filter type to bilinear and set phase to 8, for lowest spatial layer. Change-Id: Id81f4b1ba93db19c1cd37b6a46d1281a2c61bc43
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