[Cairo] New function: cairo_copy

[Carl Worth]

Some eons ago, and two mailing lists in the past,
On Nov 21, Owen Taylor wrote:
 >  * Does the stack nature of XrSave/Restore really make sense
 >    outside the context of the Postscript language environment?
 >    Conceptually they are equivalent to a copy operation,
 >    and:
 >      
 >     XrState *local = XrStateCopy (xrs);
 >     /* Do stuff with local */
 >     XrStateDestroy (locale);
 > 
 >    is not significantly harder to type than the Save/Restore operation, 
 >    and considerably easier to map onto a a "graphics context"
 >    concept. 

I've just added a new cairo_copy function that allows the coding style
described by Owen above, (which is quite useful in many situations).

I did not remove cairo_save/restore however. I think these functions
are still useful for a different coding style. (And perhaps more
concretely, I want the internal stack to be around for the purpose of
the proposed push/pop_group functionality).

I think sanity still prevails. A cairo_t still maintains a stack of
graphics state objects. However, cairo_copy does not copy the stack,
but only the current graphics state.

Does anybody see any potential for problems or confusion here?

-Carl