Added color conversion functions (rgb>spec6>rgb), based on Mario Marengo's VEX functions from his glass shader. Added optics data (Schott catalogue) for glass types, generated by modified mktables.C, also taken from Mario Marengo's glass shader. Changed the glass block so that dispersion in RYGCBV is done optionally if "usespec6" is toggled on, in which case the "ior" variable is ignored in favor of the IOR data taken from the tables, controlled by the "optics" variable (range [0,112]). Tables will get FreeSnell/Sopra n+k data later, color header will get extra color space conversions later as well.
