shell: Allow unresponsive surfaces to be moved again

Commit c85f1d45 caused the move of an unresponsive surface to be no
longer possible, since the grabbed flag would prevent the move grab
to start while the busy grab was still active.

desktop-shell/shell.c

@@ -1807,6 +1807,9 @@ set_busy_cursor(struct shell_surface *shsurf, struct weston_pointer *pointer)
 
 	shell_grab_start(grab, &busy_cursor_grab_interface, shsurf, pointer,
 			 DESKTOP_SHELL_CURSOR_BUSY);
+	/* Mark the shsurf as ungrabbed so that button binding is able
+	 * to move it. */
+	shsurf->grabbed = 0;
 }
 
 static void