Previously lookup table was used for the wavelength node, which was actually
hardcoded into the function. This could have either make compiler to require
having huge stack size or will put it into a texture memory. Either of those
ways is not good for the performance on GPU.
Replaced this lookup table with several 4-5 degree polynomial functions,
which seems to be accurate enough to give noticeable difference.
Also wavelength node without input connected is being converted to value
input at shader compile time.