 This is a C implementation of the BMRT greenmarble shader written by Larry
 Gritz. The main difference is that Renderman functions such as specular,
 ambient, etc. have been removed. This plugin will return only color values.
 It could also be noted that this shader is not actually only for green marble. 
 The controls actually allow one to create a marble with a mixture of any two
 colors plus a third color for the veins. The controls should be pretty self
 explanatory.
