This is required because upon downscaling, if a motion vector points
partially into the UMV (e.g. all minus 1 of 64+7 pixels, i.e. 70),
then we can point up to 140 pixels into the larger-resolution (2x)
reference buffer UMV, which means the UMV for reference buffers in
downscaling needs to be 140 rounded up to the nearest multiple of 32,
i.e. 160.

Longer-term, we should probably handle the UMV differently by detecting
edge coverage on-the-fly and using a temporary buffer for edge extensions
instead of adding 160 pixels on all sides of the image (which means a
CIF image uses 3x its own area size for borders).

Change-Id: I5184443e6731cd6721fc6a5d430a53e7d91b4f7e