vp9-svc: Fix to downsampling filter phase_shift.

vp9-svc: Fix to downsampling filter phase_shift.

Set phase_shift = 0 if the scale factors are
above 3/4. Removes artifact for scale factors
close to 1.

phase_shift = 8 is to get an averaging filter
(decimated pixel aligns to 8/16, midway between source pixels),
and only makes sense for scale factors multiples of
2 (1/2, 1/4,...).

Removes artifact for high scaling ratios.

Change-Id: Id0a85869d6c6156dda0032c697ded2de78fad6bd