Function append_nested 

pub fn append_nested(
    scene: &mut Scene,
    child: &Scene,
    transform: Affine,
) -> usize

scene.append(child, Some(transform)), carrying the floating surfaces drawn into child since nest_scene onto scene: what scene holds so far lies below them. Returns how many surfaces child carried. A cache that carried any must be redrawn on every recorded frame: reused, its surfaces would show the opaque material.