The Cairo clipping API is very PostScripty; you set up a path and then turn it into a clip region instead of filling it. You don't end up with a first-class value, but a manipulation of the implicit global rendering state. For CAG-style operations, http://lib2geom.sourceforge.net/ would be a better bet, but FFIing to a library written in C wouldn't necessarily be a real improvement over just writing a CAG library natively. I think it would be true to say that you can't quite get everything you would want in one easy-to-use place, then.