Deal with groups in SVG parsing

ParseSVGNode will recursively parse <svg>, <g> and <group> nodes.

Basically just going to try and implement a bunch more of SVG now.

The "transform" attribute would be a useful one to deal with because svg editors (at least, inkscape)
seem to be rather liberal in using it.

I hope the timestamp for this is correct, I ran git commit --amend but
I don't know if that will fix it.