effects/overview: Delay starting the intro animation

The ExpoLayout delays relayouting for optimization purposes. However,
this means that new geometry will be available only after returning to
the event loop.

This change delays starting the intro animation so it can be started
with new geometries.

src/effects/overview/qml/ScreenView.qml

@@ -15,6 +15,10 @@ FocusScope {
 
     property bool organized: false
 
+    function start() {
+        container.organized = true;
+    }
+
     function stop() {
         container.organized = false;
     }
@@ -118,6 +122,7 @@ FocusScope {
     }
 
     Component.onCompleted: {
-        container.organized = true;
+        // Delay starting the effect to let the window heap arrange thumbnails.
+        Qt.callLater(container.start);
     }
 }