screen: Don't use monitor indices in check_fullscreen_func

Rewrite check_fullscreen_func to not use indexes (and
offset-index-as-pointer) tricks. This also removes the usage of an API
constructing temporary logical monitor arrays carrying indices.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/core/window-private.h

@@ -731,6 +731,7 @@ void meta_window_activate_full (MetaWindow     *window,
 
 MetaLogicalMonitor * meta_window_calculate_main_logical_monitor (MetaWindow *window);
 
+MetaLogicalMonitor * meta_window_get_main_logical_monitor (MetaWindow *window);
 void meta_window_update_monitor (MetaWindow *window,
                                  gboolean    user_op);