Change radiation / slowfreeze graphics to distinguish more from other type; less from others of same type

[Nami Verasche]

http://www.lemmingsforums.net/index.php?topic=2752.0

Currently slowfreeze and radiation have several appearances; the differing structure is purely aesthetic, while the differing colors are usually what indicates the object time. Proposal is to be more consistent within an object type, and stronger differentiation between the two.