The main change is that SurfaceClosures now have the hero
wavelength baked into them. Since surface closures come from
surface intersections, and intersections are always specific to
a ray or path, and rays/paths have a fixed wavelength, it doesn't
make sense for the surface closure to constantly be converting
from a more general color representation to spectral samples
whenever its used.
This is also nice because it keeps surface closures removed from
any particular representation of color. All color space handling
etc. can be kept inside the shaders.
