layout: Rebuild struts and the input region when the Shell starts up

This means that windows will be positioned correctly with respect
to the panel when the shell starts up, and there won't be adjusting
after the session animation zooms in entirely.

https://bugzilla.gnome.org/show_bug.cgi?id=694227

js/ui/main.js

@@ -152,6 +152,7 @@ function startSession() {
     componentManager = new Components.ComponentManager();
 
     layoutManager.init();
+    layoutManager.prepareStartupAnimation();
     overview.init();
 
     global.screen.override_workspace_layout(Meta.ScreenCorner.TOPLEFT,