What sorting algorithm(s) is this?  Is there a way to take advantage of frame-to-frame coherence?  That is the sort order is unlikely to change, or change much, between frames.

Also, let's wait until we need it, but I see us adding a `Scene.sortTranslucentPass` flag for apps that don't need this, e.g., if all the translucent objects are on the surface, the sort doesn't help.