1/2/98: refract-Beta

Refract is back, and better than ever!  That's right...  The New Layer 
checkbox now works properly from layers with and without alpha.

Fixed so small or goofily placed lenses don't crash.  Lo and behold,
lens offsets also seem to be working.  How that happened is beyond me.

Fixed some bad GParam values thing in run().

Changed the name of the parameter that determines the lens thickness
to "thick" instead of "depth".
