xwayland: add a more reliable way to detect Xwayland
Some X11 clients (e.g. Mesa) need to check whether the X11 server is Xwayland. Add a root window property indicating this. Should be more reliable than checking RandR output names, especially if we want to mirror Wayland wl_output.name in the future.
Signed-off-by: Simon Ser contact@emersion.fr
cc @daenzer