vp9: Add phase to get averaging filter for 1:2 downsampling.
The scaling filter with zero shift will give sub-sampling for 2x downsampling. Allow for a phase shift to get an averaging filter. Usage is for source scaling in 1 pass SVC mode for 1:2 downscale. Reduces aliasing in downsampled image. Keep the phase to 0/off for now. Change-Id: Ic547ea0748d151b675f877527e656407fcf4d51e
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