[cairo] pattern/glitz update merge

[David Reveman]

I did some work today to get my updates to the pattern system and to the
glitz backend merged with current cvs. The patch is actually too big for
the list so I put it here:
http://www.cs.umu.se/~c99drn/cairo-pattern-merge-1.diff

It modifies the new fall-back system slightly. I moved some of the
fall-back stuff into the cairo_image_surface struct, it cleaned up
things quite nicely and made the merge a lot easier. 

Another thing that might be of concern is that I've done some much
needed restructuring of the internal cairo_pattern struct, and it's now
a union instead of a struct, which meant I also had to change the
typedef in cairo.h. I don't see any problem with this, but maybe someone
else does?

-David