[cairo] Current XCB backend breakage compared to Xlib

[Ian Osgood]

On my machine, the current head has the following breakage  
differences between XCB and Xlib. I'm testing on Mac OS X against  
Apple's XFree86 4.4 server.

XCB Fails, Xlib passes:

get-group-target (no images generated)
paint-with-alpha (corrupt images)
text-antialias-none (corrupt images)
text-antialias-subpixel (nearly correct images)

Xlib XPASS:

a8-mask

About half of the tests fail for both X backends.

Ian