I don't know what the standard way to submit patches is, but I've attached a patch that implements loading, saving, and computing of the clip-rule property.
I'm playing with getting it to work in the cairo renderer. It's a bit of a pain for two reasons. First, the existing fill-rule implementation for clip paths is not just incorrect (fill-rule shouldn't affect clip paths) but is also very buggy. Second, I don't know how to ask cairo to generate a clipping region that is the union of two paths unless both paths use the nonzero rule. That being said, I have a gross hack that at least gets multiple nonzero paths or a single evenodd path right. I'll make it less ugly before I mail it in.
I understand approximately nothing about the nr renderer (or whatever the code that I assume renders the editing view is), so I've made no progress on honoring clip-rule in the edit view.
What should I do from here?
(Note: I'm not really inclined to try to invent a sensible UI for clip-rule. I mainly want to be able to edit my own figures that need clip-rule=evenodd.
--Andy