Fix SVGMarker for 0.8

In case you haven't already taken a look at the SVG marker stuff, this patches it to work with 0.8.  I'm not sure about a couple of things in here:

* should the `width` and `height` be set on `g` instead of `svg`?
* can we avoid disposing of and recreating the `g` every time `getPathString` is called?  It seems safest to remove and add, but I don't know what the performance implications are.