loginDialog: hide authprompt when VT switching away

The authprompt is no longer useful if the user isn't
on the VT, and it's not what the user is going to
ultimately see when they come back (they'll see the
user list), so we should hide it.

Fixes a bug where it briefly blinks in view on
VT switch.

js/gdm/loginDialog.js

@@ -940,6 +940,9 @@ var LoginDialog = GObject.registerClass({
             this._greeter.run_dispose();
             this._greeter = null;
         }
+
+	this._authPrompt.hide();
+	this.actor.opacity = 0;
     }
 
     _gotGreeterSessionProxy(proxy) {