EPIC: XWM testing

Weston has a built-in X11 window manager (XWM). Weston test suite has exactly one test touching Xwayland: a smoke test that only checks if Xwayland started.

XWM is a hard thing to develop. You have to know both the old X11 world and Weston's internals including libweston-desktop and desktop-shell. XWM is also making blocking X11 roundtrips to Xwayland, and Xwayland makes Wayland calls to Weston, so the danger of deadlocking is obvious, although mostly relying on Xwayland never making a blocking Wayland roundtrip. XWM must handle two asynchronous protocol streams simultaneously and correlate between them: Wayland and X11. The X11 window management side is full of arcane magic, forgotten specs, and races.

All that makes XWM extremely fragile, and we have no automated tests to ensure even the simplest cases work correctly, not to mention the more "interesting" ones that decades of badly written X11 apps have imposed on traditional X11 window managers.

Figure out how we could get XWM properly tested e.g. with screenshot-based tests.

I would not like to introduce a huge amount of X11 programs in the Weston repository, but then again is there any other option to exercise XWM in a controlled fashion?