Allow visibility MBR to be manually offset

This patch adds internal properties to "2D" that indicate how far beyond an object's default dimensions the visible MBR should be.

These can be set with the public method `offsetBoundary`.

The point of this is that custom rendering might want to create effects that extend beyond the standard dimensions of the entity -- a glow around a sprite, for instance.  It shouldn't really be used anywhere *except* custom rendering stuff.