[cairo] Workaround for bug 22484. Please test the 1.6 X server with pixman master

[Soeren Sandmann]

Hi,

I just pushed a workaround for 

        https://bugs.freedesktop.org/show_bug.cgi?id=22484

to pixman master.

What's going on in the bug is that the server is creating a pixman
image pointing to the wrong place in memory, but then setting a clip
region pointing to the right place. Because older versions of pixman
would not clip against image geometry, this would actually work.

So basically a bug in the X server is cancelling out a bug in
pixman. Both bugs were introduced by me, so it's only fair that I had
to deal with them.

It is quite tricky to fix because what the server is doing is legal,
but the correct result is not what the server actually wants. The
workaround tries to detect when the server really wants the incorrect
result.

I believe the workaround is correct and it seems to work for me, buf
if it turns out it isn't, the result could be crashes or corruption.
So if you can, please test pixman git master with a 1.6 X server and
report any irregularities.

If no bugs are reported, I'll make a new development release of pixman
early next week.


Thanks,
Soren