Use canvas for drawing

[rossjrw]

Canvas is probably a lot faster to draw than SVG. I think I should have been using it all along, honestly.

I should save SVG for the export. The preview should be canvas.

[rossjrw]

No need to implement this yet - though I do think it should be done eventually - but having looked at the API I think I'd prefer to avoid using a framework for this. The syntax is fairly similar to what I'm already doing for SVG. I should be able to write a method that takes a general source of structural information and produces either a canvas path or an SVG path.