[Finishes #81859562] use canvg for comparison

Convert the svg to a canvas, extract the pixel data, and use that to compare the user's answer with the expected answer.

Required a few hacks to get canvg working across various environments. I've validated on node, chrome, safari, ff, IE 11. I'll have to be sure to check IE9 once it's up on staging.
Please make sure this doesn't break Functional blocks on level  /2014/11 ... I know, I should have tests