Cairo specific PS creation

[Fred Kiefer]

I just committed some code to use cairo functionality for the creation
of Postscript files. In the long run I expect this to be a lot better
then our own grown way of PS creation. But for now there are a few
severe limitations to this code:
- Now show page support. This will result in all drawing ending up on
the same page. The solution is obviously to add a showpage command for
the graphics output. This would be a nop for most graphics contexts, the
GSStreamContext would output "showpage" and the cairos PS surface call a
method that is already in my code.
- Old cairo libraries don't flush when closing a surface. This results
in the PS output file being almost empty. This is resolved by
implementing the showpage command or by upgrading to a newer cairo
release. (Or as a hack, add a call to show page when closing a CairoGState)
- The page size is currently set to hard coded 400x400 points. This size
should be set from the method [NSView beginPage:label:bBox:fonts:].
Again a new method on the graphics context is needed here.
- All GNUstep comments that get send to the PS output are ignored. Not
sure, if this is a bug or a feature :-)

Hope you have fun to play with this new functionality.

Fred