[cairo] mac patches for review

[Baz]

http://gitweb.freedesktop.org/?p=users/brianewins/cairo;a=shortlog;h=quartz-fixes3

Not as interesting as the other patches this week - this series of 6
fixes up all the instances I could find of us ignoring the return
values in the quartz and atsui backends - a low-rent version of the
work Chris Wilson did for the rest of the code.

I was prompted to do this because one of the bugs causes a crash in
firefox, but on the way found a couple of leaks and recursive use of
the scaled-font mutex. The last patch is the one I'm least happy with,
where I'm just returning NULLs after I get an error. Comments
appreciated.

Cheers,
Baz