[cairo] enumeration of backends

[David Reveman]

On Thu, 2004-05-20 at 14:17 +0200, Øyvind Kolås wrote:
> 
>  another thing that would be useful, is choosing which backend actually
>  feeds the image backend, and thus the png backend, this could be used
>  to hardware accelerate creation of png images on a webserver.

This is a great idea!

However changing the backend which feeds the image backend feels a bit
dangerous. e.g. what if the GL backend feeds the image backend and the
GL backend needs to fall back to the image backend for some operation.

Making it possible to control which backend that feeds the png backend
would be enough.
 
-David