Improve SVG coordinate system.

The method for getting the SVG's position on the page (to transform
event coordinates to local SVG coordinates) used
svg.getBoundingClientRect(), which produces unreliable results in
Firefox. This has been replaced with svg.getScreenCTM(), which is
known to work better in a variety of browsers.

	modified:   svgwidgets.js
	modified:   svgwidgets.py