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>

hw/xwayland/xwayland-screen.c

@@ -635,6 +635,11 @@ xwl_screen_init(ScreenPtr pScreen, int argc, char **argv)
                              &registry_listener, xwl_screen);
     xwl_screen_roundtrip(xwl_screen);
 
+    if (!xwl_screen->rootless && !xwl_screen->xdg_wm_base) {
+        ErrorF("missing XDG-WM-Base protocol\n");
+        return FALSE;
+    }
+
     bpc = xwl_screen->depth / 3;
     green_bpc = xwl_screen->depth - 2 * bpc;
     blue_mask = (1 << bpc) - 1;