Bug 711329 - Artifacts on crop tool's passepartout when set to "allow growing"

Calculate GimpCanvasPassepartout's extents using a cairo_region_xor()
of the crop rectangle and the image extents, instead of a
cairo_region_subtract(). This fixes the artifacts and has the nice
side effect that the passepartout now colors both the parts of the
image that will be removed *and* the parts of the image that will be
newly added.