The init func does not actually need to draw anything, but having a cairo_t
similar to that passed to render_glyph is handy for computing font extents.
This is because cairo makes doing some things really hard (if not impossible)
without a cairo_t.
The user-font-proxy test case is a great example of how the added cairo_t
makes life much easier.
This way the same callback code can be used to render multiple different
glyph arrays. Change done for education purposes, otherwise doesn't
make any difference in the test.
