not GimpCanvasLine* etc. We keep them around as items, and their
constructors return items, and it's getting more code anyway when
porting away from pause/resume, so let's keep the casting minimal at
least.
Instead, keep around the created GimpCanvasItems, and update them when
the blend coordiates change. Add setters to GipmCanvasLine and
GimpCanvasHandle which take care of calling begin_change() and
end_change() on the items around the change, so thes invalidate
properly.
Michael Natterer