[cairo] semantics of cairo_copy()

[Jonathan Brandmeyer]

What are the semantics of cairo_copy()?  Specifically: What parts of the
state are copied?   What preconditions must be met for src and dest
before calling cairo_copy()?  Can a cairo_t targeting a PNG file be
copied to a cairo_t targeting X11?  Is it an error to perform a
cairo_copy() to a cairo_t that has not been targeted from one that is?**

What usage model is cairo_copy intended for?

Thanks,
-Jonathan Brandmeyer

** This one is an error right now: the code is "SIGSEGV" ;)  At worst,
it should set the error flag.  Is it possible to make this work at all?