1) We need to add Z-buffering to JavaFX for sorting objects by depth when using 3D transforms.

May consider adding the ability to enable Z-buffer on Node. This will allow the support of underlay, overlay and depth sorted rendering. 

2) Resolve semantics of mixing Z depth sorting with rendering order.

The current "depth" order for 2D objects is a strict painters algorithm where the last thing rendered ends up on top. There are scene graph operations to send a node to the front or back of the Z stack by moving it to the end or beginning of the rendering order. We need to rationalize this with 3D depth sorting (e.g., with a Z-buffer), taking into account transparency.