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>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
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