xwayland: non-rootless requires the XDG-WM-Base protocol
When running non-rootless, Xwayland requires that the Wayland compositor supports the XDG-WM-Base protocol.
Check for XDG-WM-Base protocol support at startup and exit cleanly if missing rather than segfaulting later in ensure_surface_for_window() while trying to use xdg_wm_base_get_xdg_surface().
Signed-off-by: Olivier Fourdan ofourdan@redhat.com