compositor-x11: Improve fullscreen support

compositor-x11: Improve fullscreen support

The old implementation didn't work because we set the minimum and maximum
sizes so that the WM can't resize us.  That makes the fullscreen protocol
not work.  Additionally we were requesting fullscreen after mapping, which
requires the more complicated (and potentially flickery) client message
approach.

Now we just set the _NET_WM_STATE before mapping and avoid setting
the size hints in case of fullscreen.  That's all good, but the problem
is that we now have to wait for configure notify before we know
what size our output will be.  For now we just block and pull events from
X until we get the size.

Ideally we would treat the map as an output hotplug event and just add the
output at that point, but we can't start up with no outputs present.
That may be worth fixing, but for now, the block-on-map is fine.