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.
and get the shell from gimp_canvas_item_get_shell() if needed. Remove
some cairo_translate() that didn't have any effect and were leftovers
from a very early canvas item code.
