[cairo] Quartz backend fix

[Andrea Canciani]

I did some more work on the quartz backend and I got blending *mostly* working.
As a side effect, I think that now SATURATE is handled correctly (by
falling back).
Please notice that the third patch has only been tested on Leopard
(10.5) and I'm not even sure if it compiles on Panther and Tiger (it
should work correctly on Tiger and not compile on Panther, but there
might also be some other problems with Panther in the current code).
Is there anybody able to try these on 10.3?
Expected changes in the test suite:
- clip-operator should FAIL (it had a special ref image for quartz to
ignore the SATURATE problem)
- operator and operator-alpha should PASS
- extended-blend and extended-blend-alpha should FAIL, but resultimg
image should be very similar to reference (except for 2 blend modes in
extended-blend-alpha.rgb24... I still have to find what's wrong
there).
Please review and test before committing (as you already have noticed
I'm quite new to cairo internals)
Thank you
Andrea
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