vp10: allow MV refs to point outside visible image.
In VP9, the ref MV had to point to a block that itself fully resided within the visible image, i.e. all borders of the image had to be within the visible borders of the coded frame. This is somewhat illogical, and had obscure side effects, e.g. clamping of fairly reasonable motion vectors such as 0,0 were clipped to negative values if the block was overhanging on frame edges (such as the last rows on 1080p content), which makes no sense whatsoever. Instead, relax clamping constraints such that the ref MVs are allowed to point to blocks exactly outside the visible edges in both Y as well as UV planes, including the 8tap filter edges (that's why the offset is 8 pixels + block size). See issue 1037. Change-Id: I2683eb2a18b24955e4dcce36c2940aa2ba3a1061
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