Testing of modes with Alt Ref frame

Previously when a frame was being overlaid on a previously coded
alt ref frame we only checked the alt ref 0,0 mode. Where there is
a possibility that the alt ref buffer is a filtered frame we should allow
the other prediction modes as normal or at the least allow use of
the last frame buffer.

Change-Id: I4d6227223d125c96b4f3066ec6ec9484fee7768c