[cairo] make return buffers more obvious

[Lawrence D'Oliveiro]

On Fri, 18 Dec 2015 21:23:13 +0100, Enrico Weigelt, metux IT consult
wrote:

> ... because somewhere, somewhen, we'd
> like to change the object our passed pointer points to (but not in
> that function itself).

E.g. because that object contains a cache inside it that the
caller doesn’t know about, that the functions have to maintain.

> const simply can't express that (actually, I dont know any language
> that can do that), so we'd need another way of describing this.

I believe C++ has mutable const for this sort of purpose ...