[cairo] [PATCH] image: Avoid reusing pixman images for threadsafety.

[M Joonas Pihlaja]

I'd like to push this commit to master and 1.10:

http://cgit.freedesktop.org/~joonas/cairo/commit/?h=no-pixman-ref&id=af403a204a88d882317a3e91abe8f0cc454852a4

It fixes the pthread-* tests by avoiding sharing of pixman_image_t
objects and thus not relying on atomic ref/unref semantics in pixman.
Applying this gives us the same threadsafety guarantees that 1.8 has
in practise for the image backend.  We can easily revert it later when
pixman does get atomics.

Cheers,

Joonas