Detail:
  * Draw paths can be clipped against other Draw paths using an additional
    pair of SWI calls. Draw path clipping is by remembering the scan lines
    that would be used for the clip path and then clipping against those
    when plotting the draw path. Ick.
Admin:
  Very experimental, very nasty, but it works and does do the clipping.
  Can be conditionally removed.

Version 1.13. Tagged as 'Draw-1_13'