Clearly what is needed is to add rectangles and carve out rectangles giving a polygon result which can
be drawn as an SVG path.

This requires a polygon package...  it looks like gpc.c is the most approachable one - single C file.

We don't need curves and probably not bloating/shrinking etc for this application.

This is a bigger project than what is being tested here but should only be needed if there are 
problems with the laser cutting, eg when going over the same line twice.

In a proper poly system, nested sheets need to be cut from the deepest layers back so that wood does
not drop out from the sheet before nested holes are cut.