And that concludes our rework of reference cylce detection. All code
that references other objects via IRIs will now call rsvg_acquire_node()
which itself avoids giving out the same node a second time until
rsvg_release_node() has been called.

This also means that rsvg_defs_lookup() is only used in two places now:
(1) by rsvg_acquire_node()
(2) by public API that allows operating on objects by id.