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/main.js

@@ -89,6 +89,9 @@ function start() {
     global.logError = window.log;
     global.log = window.log;
 
+    // Hide the stage until we're ready for it
+    global.stage.hide();
+
     // Chain up async errors reported from C
     global.connect('notify-error', function (global, msg, detail) { notifyError(msg, detail); });