GUI/back changes in CVS

[Fred Kiefer]

I just added changes to GUI and back that will in the end allow pattern
images for GNUstep and the new MacOSX composite operator that uses an
addtional alpha value. I wanted these changes to go into the next
release as this will break binary compatibility between GUI and back. I
have tested this change with xlib. It would be great, if somebody could
test it before the release with art and winlib.

I hope to finish the support for pattern colours over the weekend. What
I wont eb working in is new composite operator for the art backend. The
win backend had this already implemented for some time.

Cheers
Fred