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.
I couldn't make the BVH4 faster than the BVH, and the bitstack
was bloating the AccelRay struct. Removing the bitstack gives
a small but noticable speedup in rendering.
BVH traversal still happens in local space, but final actual
surface intersection calculations are done in world space by
transforming the triangle into world space. This is to improve
numerical consistency between intersections.
