[cairo] some mac fixes pushed

[Baz]

Jesse Ruderman pointed out this morning that I hadn't pushed a fix I
had for a crasher in firefox, so I cleaned up the quartz-fixes3 branch
and got most of them pushed.

I've still got a few things held back:
- various fixes that added reference images; these were controversial
when I made them because they were asserting that the quartz rendering
was ok in some cases where it differed noticably from the image
backend, particularly for dashing.
- 'fill with alpha for EXTEND_NONE': this fixed the operator-source
test but regresses the self-copy test
- add _cairo_quartz_surface_mask_with_pattern: is probably ok to go
but was sitting later in my queue than the extend_none fix, rather
than swap them and fix up the conflicts I'll try to get both working.