xdg-shell: Fail xdg_surface creation when surface has role

If a given wl_surface already has a role (e.g. cursor or subsurface),
there is nothing you can do with an xdg_surface which won't raise an
error, apart from destroying it.

As of wayland/wayland-protocols@11fecf080860 this is now explicitly
specified to be illegal, so disallow it within libweston-desktop. This
avoids us tying ourselves in knots with surface-private ownership.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston-desktop/xdg-shell.c

@@ -1422,6 +1422,13 @@ weston_desktop_xdg_shell_protocol_get_xdg_surface(struct wl_client *wl_client,
 		wl_resource_get_user_data(surface_resource);
 	struct weston_desktop_xdg_surface *surface;
 
+	if (wsurface->committed) {
+		wl_resource_post_error(resource,
+				       XDG_WM_BASE_ERROR_ROLE,
+				       "xdg_surface must not have any other role");
+		return;
+	}
+
 	if (wsurface->buffer_ref.buffer != NULL) {
 		wl_resource_post_error(resource,
 				       XDG_SURFACE_ERROR_UNCONFIGURED_BUFFER,