Provide a way to specify extra CSS information

Librsvg's public API doesn't let the caller specify extra styling information when rendering an SVG. People use various hacks to accomplish this, which boil down to creating a wrapper SVG with a <style> element, and then somehow including the original SVG under this. For example, GTK uses an xi:include element with some precarious escaping to construct a data: URL.

Similarly, we had a bug about Wikipedia wanting to use fill="currentColor" when using equations rendered as SVG; the intent was to have the equations be rendered in the same color as the surrounding text.

It would be useful to have a way to specify extra stylesheet data when rendering an SVG.