vp9: SVC: Set downsample filtertype for lowest spatial layer.
vp9: SVC: Set downsample filtertype for lowest spatial layer.

For lowest spatial layer, in 3 layer SVC, set the
downsampling filtertype to get averaging filter.
Needed for reducing aliasing on low-res layer,
small increase in overall encoder time.

Change-Id: Ia31460123bd91b72eca49b46dd924b9f226d4563

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