main: put noise texture below startup animation, not still frames

Right now we take a still frame of the desktop before showing the
start up animation.  This gives us an animation over what was
there before startup.

That's not actually desirable when restarting the shell.  We don't
want to animate over undecorated windows, we really want to animate
over the noise texture.

This commit drops the still frames in favor of the noise texture.

https://bugzilla.gnome.org/show_bug.cgi?id=694326

js/ui/main.js

@@ -196,13 +196,10 @@ function startSession() {
     ExtensionDownloader.init();
     ExtensionSystem.init();
 
-    // Run the startup animation as soon as the mainloop is idle enough.
-    // This is necessary to have it smooth and without interruptions from
-    // completed IO tasks
-    GLib.idle_add(GLib.PRIORITY_LOW, function() {
-        layoutManager.startupAnimation();
-        return false;
-    });
+    layoutManager.connect('startup-prepared',
+                          Lang.bind(this, function() {
+                              layoutManager.startupAnimation();
+                          }));
 }
 
 let _workspaces = [];