[cairo] New OpenGL backend merged

[Chris Wilson]

I've have pushed Eric's OpenGL backend to master [1]. My opinion is that
this was the most suitable basis for building a new high-level GL
backend. It almost entirely passes the test suite [2] and we should aim
to keep the test suite passing in each future step.

At the moment the acceleration is most basic and cairo-perf-trace
suggests not to get your hopes up too early. The next step I have in
mind is to merge the clipping rework which should dramatically boost
this backend's performance in real-world scenarios. After that, I'm
looking at including cairo-drm and sharing some of the code there (in
particular the glyph cache).

Of course, everyone is invited to come and play -- and improve -- the
new backend! Let's have fun improving Cairo.
-ickle

[1] Still marked as experimental, so pass --enable-gl to configure.

[2] Here there are just two failures, a sampling error in text-antialias
and a crash in gradient-zero-stops which appears to be a symptom of
keeping the GLX context alive past the call to XCloseDisplay().