Call backend show() in gdk_window_show even if window is mapped

This is required for the GtkSocket code, as it shows the plug child
even though the current cached state is (wrongly) that its already
mapped.

This makes blink work for non-local case in testsocket.

gdk/gdkwindow.c

@@ -5497,7 +5497,7 @@ gdk_window_show_internal (GdkWindow *window, gboolean raise)
       private->state = 0;
     }
 
-  if (!was_mapped && gdk_window_is_viewable (window))
+  if (gdk_window_is_viewable (window))
     show_all_visible_impls (private);
   
   if (!was_mapped)