Running cairo's tests on xwayland under weston

[darxus at chaosreigns.com]

SardemFF7 suggested running cairo's make check against xwayland under
weston, which sounded fun.  

Cairo's tests have some difficulties, but this much passes normally:
https://lists.cairographics.org/archives/cairo/2016-August/027718.html

Doing the same on xwayland also passes, which is cool.  But testing against
freshly generated references skips a lot of usefulness.

Then I ran the tests on xwayland with the references generated with Xvfb
(which is what cairo recommends).  Most of those passed, with the exception
of tests for xcb-window output.  Most of them failed, all with blank output
(all black).  It looks like all the xcb-window tests that passed may have
been expected to have blank output. 


Anybody have thoughts on why that output would be all black?  Or if it's a
problem that should be fixed?  On the wayland or cairo side?


(I ran weston with the -i0 command line argument to disable the screensaver
/ locking.  And I ran weston as an X client.)