Add a retained engine layer subtree from previous frames.
All the engine layers that are in the subtree of the retained layer will be automatically appended to the current engine layer tree.
Therefore, when implementing a subclass of the Layer concept defined in
the rendering layer of Flutter's framework, once this is called, there's
no need to call
addToScene for its children layers.
EngineLayer addRetained(EngineLayer retainedLayer) native 'SceneBuilder_addRetained';