Set and use uv_crop_[width|height]

Ensure consistent border extension by rounding uv_crop_* at image
creation time. Where it was rounded problems could arise with the right
and bottom extensions.
When padding = 32, y_width = 64, and y_crop_width = 63:
 (padding + width - crop_width + 1) / 2
 32 + 64 - 63 + 1 should equal 32 *but*
 32 + 1 + 1 equals 34 giving a right buffer of 17 instead of 16.
By calculating uv_crop_* earlier we round up at the appropriate time and
for the same values:
 (y_crop_width + 1) / 2
 63 + 1 / 2
 64
 (padding / 2) + uv_width - uv_crop_width
 16 + 16 - 16
 16

Change-Id: If866cd1b63444771440edb1432280ac83875969b