svp clipping: Rectangle dimensions don't map directly to B2IBox.

Previously the clip region would end up being 1 pixel smaller in both
directions than the requested area -- B2IBox considers the bottom-right
point to NOT be included in its area, whereas Rectangle considers the
bottom-right point to be the last point WITHIN its area.

(This is equivalent to the Rectangle's getWidth/GetWidth differentiation.)

This is noticeable e.g. with tiled rendering, where images spanning
a tile boundary are clipped to paint only the region within a given
tile -- however previously they would be clipped by an additional pixel
in each direciton, resulting in an empty border along the bottom-right
edge.

Change-Id: I0ef3098959defc5c82719155c130aa904c04b6b1