Improve icon area invalidation
This unfortunately somewhat only papers over a redraw bug, and also unfortunately likely causes us to draw a little more than is strictly necessary, but should fix some redraw glitches when removing/moving an icon that has a wide pixbuf.
At some point I should figure out why the pixbuf extents seems to be 1 pixel (at least) larger than the cacluated extents, but for now this will do.
Closes #215 (closed)