I've recently been looking at using libcroco from Inkscape. I'll need some help in hooking it up properly (e.g. ensuring that the picture updates whenever stylesheets in the document are modified), but at least I can do the main work of interfacing with libcroco.
As a result of looking at libcroco, I've noticed a few bugs, so I'm working on fixing those and updating the test suite so that it catches those bugs.
I've also noticed that there are a number of things about style that aren't specified in the SVG spec, e.g. the priority of style="..." attributes compared to other stylesheet information. My guess is that it's considered more specific than any separate stylesheet rule.