[cairo] Cairo and ISO C++

[Bill Spitzak]

I think it would be better to support that with a special "object id" 
cairo backend.

Dov Grobgeld wrote:
> Please don't even consider taking away anti-aliasing. I'm using it in 
> [1] and [2] in order to use the same graphical primitives for painting 
> and for outputting a label image (an image where the  "color" indicates 
> a label for the object at the pixel). Painting with antialiased graphics 
> destroys the ability to create the label image.
> 
> See:
> [1] https://github.com/dov/dovtk-lasso
> [2] https://github.com/dov/giv