This disables retained rendering for Leader/FollowerLayer.
A FollowerLayer copies changes from a LeaderLayer that could be anywhere in the Layer tree, and that LeaderLayer could change without notifying the FollowerLayer. Therefore we have to always call a FollowerLayer's addToScene. In order to call FollowerLayer's addToScene, LeaderLayer's addToScene must be called first so LeaderLayer must also be considered as alwaysNeedsAddToScene.
@override bool get alwaysNeedsAddToScene => true;