How do I use cairomm and librsvg?

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I have a C++ program that uses cairomm, and I'd like to use librsvg with 
it to render SVG files to Cairo surfaces. However, librsvg is in C, and 
rsvg_handle_render_cairo expects a cairo_t value, while cairomm's 
Cairo::Context doesn't give access to the underlying C type (it's 
protected). I tried subclassing Cairo::Context and implementing a 
function that returned a pointer to the underlying cairo_t type, but it 
doesn't seem to be working.

Code is as follows:

main.cpp: https://paste.gnome.org/p9fzmosnu
Makefile: https://paste.gnome.org/p6qswmxgr
mor.glade: https://paste.gnome.org/phlvgqqt5

It just prints
(mor:7839): librsvg-CRITICAL **: rsvg_handle_render_cairo_sub: assertion 
'handle != NULL' failed
repeatedly, and doesn't render anything.

Any help would be appreciated.