[cairo] How to export image_surface pixel data?

[mental at rydia.net]

Quoting Vladimir Vukicevic <vladimirv at gmail.com>:

> The internal backend methods acquire_dest_image and
> release_dest_image already have this exact API; I'm not sure
> whether it should really be exposed, though.

{acquire,release}_dest_image avoids blitting if the original surface
is an image?

> It's possible to get the exact same semantics by just creating a
> temporary surface of your width/height, drawing your original
> surface to it, doing whatever munging you want, and drawing
> back...

Yeah, that's the thing; I'd like to avoid those two blits when it's
possible to do so.

-mental