[BZ#620124] allow overriding href resources
Submitted by Christian Persch
I want to be able to provide extra xinclude data from memory, instead of from file: or gvfs locations. (I'm going to use this to provide runtime stylesheets etc.)
To do this, I need to redirect href resource lookup through the RsvgHandleClass (which I can then derive).
Proof-of-concept patch attached.