Clean up qtk.

example-app/examplescene.cpp

@@ -58,8 +58,11 @@ void ExampleScene::init()
 
 void ExampleScene::draw()
 {
-  // No custom draw logic for this example.
+  // The base class method _must_ be called first, before additional logic.
   Scene::draw();
+
+  // No additional custom draw logic for this example.
+  // QtkScene in Qtk desktop application is an example using custom draw logic.
 }
 
 void ExampleScene::update()