[cairo] Implementing cairo_read_func_t/cairo_write_func_t properly

[Lawrence D'Oliveiro]

On Sun, 3 Jan 2016 18:37:25 +0100, Enrico Weigelt, metux IT consult
wrote:

> Yeah, but I wonder, what's the benefit of streaming here at all.

I should have thought it would be blindingly obvious why callbacks are
such a wonderful idea for code reuse: because they minimize assumptions
about the caller’s data structures.