normalize html canvas for retina displays

gistfile1.txt

// from https://github.com/subprotocol/verlet-js/blob/master/examples/shapes.html
  	var dpr = window.devicePixelRatio || 1;
		canvas.width = width*dpr;
		canvas.height = height*dpr;
		canvas.getContext("2d").scale(dpr, dpr);