marker: Resolve lazily
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.
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