[cairo] Implementing cairo_read_func_t/cairo_write_func_t properly

[Lawrence D'Oliveiro]

On Fri, 01 Jan 2016 10:32:59 +0100, Bernhard Fischer wrote:

> cairo_..._stream() are stream reading functions. This is that it does
> not know in advance how many bytes are available in the file.

That’s not how they work. Cairo uses them to read known file formats.
If it doesn’t get the number of bytes it expects, then there is
something wrong with the file.