main: don't show stage until still frames are loaded

We sometimes map the stage before we've loaded a background on it
because of a race asynchronously loading the session mode.

This manifests as the startup animating starting over a white
background.

This commit defers showing the stage until after the still frames
are loaded.

https://bugzilla.gnome.org/show_bug.cgi?id=694321

js/ui/layout.js

@@ -573,6 +573,7 @@ const LayoutManager = new Lang.Class({
     },
 
     startupAnimation: function() {
+        global.stage.show();
         if (Main.sessionMode.isGreeter)
             this._startupAnimationGreeter();
         else