Allow ambient shadowing on transparent surfaces

[eustermann@…]

Would like ambient shadowing on transparent surfaces. Currently transparent surfaces do not cast ambient shadows because this makes the atomic model inside the surface black or dark. A possible solution is to allow the surface to cast shadows but allow setting the molecular model inside the surface to not receive shadows. A problem with this is that the user would probably have to know some obscure command to disable receiving shadows on the interior model. It seems to tricky to try to figure out which models are inside a transparent surface and automatically change shadow behavior.