This callback is used by the engine to notify applications that
loading of low-level ressources has been done and all internal
initalisation has finished.
Implementation of yrqApplication engineRenderCallback: After rendering each
3D frame, the engine instance calls this callback to allow for the application
to do 2D overlay rendering.
Implementation of yrqApplication engineUpdatCallback: The demos high-level
control logic is implemented here, and executed continuously by the engine
instance.