Deprecate rsvg_init and rsvg_term

rsvg_init() should be replaced by g_type_init(); we now call xmlInitParser()
in RsvgHandleClass::init.

rsvg_term() was dangerous to call, so just make it a no-op and add a new
rsvg_cleanup() that calls xmlCleanupParser(), for use only when you know
what you're doing.

https://bugzilla.gnome.org/show_bug.cgi?id=614157