Better color model in 3D

[bergey]

Right now the POVRay backend reuses FillColor. We should instead have native 3D attributes. Most of the work here is probably in surveying other 3D software so we don't end up reproducing the POVRay color model at the expense of compatibility. Possibilities include

• old (fixed function) OpenGL
• Radiance

At minimum, we should have a base color and some measure of highlight intensity.