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.c

@@ -3548,6 +3548,12 @@ meta_window_get_monitor (MetaWindow *window)
   return window->monitor->number;
 }
 
+MetaLogicalMonitor *
+meta_window_get_main_logical_monitor (MetaWindow *window)
+{
+  return window->monitor;
+}
+
 static MetaLogicalMonitor *
 find_monitor_by_winsys_id (MetaWindow *window,
                            guint       winsys_id)