[cairo] Extending Cairo API to support reusable paths

[M Joonas Pihlaja]

>     /*
>      * Creates a backend specific path object from the current path data
>      */
>     const void *
>     (*build_path_object)    (cairo_path_fixed_t     *path,
>                  cairo_gstate_t         *gstate);
[snip more void pointers]

This gives me pimples.  The returned pointer type ought to be typed, 
so that the responsibility of keeping track of types is with the 
implementor rather than the user of these APIs.  It's okay to be a 
completely opaque type imho, but not one that says "whatevah, I do 
what I want!" one.

Cheers,

Joonas