WaveFrontReader MTL support for xyz color-space values

[walbourn]

The MTL reader currently only supports RGB values for the colors. There is a variant which writes the colors in the CIE XYZ color-space instead.

Ka r g b
Ka xyz x y z

Kd r g b
Kd xyz x y z

Ks r g b
Ks xyz x y z

There is also a "Transmission Factor" color I don't handle, which I should at least indicate in a debug build.

Tf r g b
Tf xyz x y z

And a Spectral Curve File (.rfl) color option which I don't need or support, but should also document:

Ka spectral file.rtl
Ka spectral file.rtl factor

Kd spectral file.rtl
Kd spectral file.rtl factor

Ks spectral file.rtl
Ks spectral file.rtl factor

Tf spectral file.rtl
Tf spectral file.rtl factor

And finally I note that the 'dissolve' (i.e. alpha) has an optional switch -halo which I don't currently handle.

d factor
d -halo factor

See MTL OBJ materials file.