and as a "side effect", speed up rendering the image significantly:

- disable double buffering on the canvas widget.
- implement background clearing ourselves (needed after turning off
  double buffering).
- remove any fiddling with clipping regions on the tool drawing GCs
  and pull the pause/resume code out of the actual image expose
  function.
- if there are overlay widgets on the canvas, implement double
  buffering manually to aviod flicker, but do it in a way that keeps
  pausing/resuming the active tool *outside* the double buffered
  drawing.