backends/dummy: Set scale of dummy output

To make the nested compositor mode work again after "backends/native:
Calculate the output scale in here", set the scale when creating the
dummy output.

https://bugzilla.gnome.org/show_bug.cgi?id=745401

src/backends/meta-monitor-manager-dummy.c

@@ -96,6 +96,7 @@ meta_monitor_manager_dummy_read_current (MetaMonitorManager *manager)
   manager->outputs[0].backlight_min = 0;
   manager->outputs[0].backlight_max = 0;
   manager->outputs[0].connector_type = META_CONNECTOR_TYPE_LVDS;
+  manager->outputs[0].scale = 1;
 }
 
 static void