update method
Advances the scene by deltaSeconds: ticks every node's components
and animation players, and refreshes the flat render layer.
Calling this is optional. A caller that only calls render gets an implicit tick with a wall-clock delta. Call update explicitly to drive the scene with a fixed or supplied timestep, then call render; the render then skips its implicit tick.
Implementation
void update(double deltaSeconds) {
_tick(deltaSeconds);
_tickedThisFrame = true;
}