test:
- object motion blur
- camera motion blur
- launching pbrt.exe?
- all materials

bugs:
- output image filename doesn't end with exr
- change to pretend that metropolis is just a surface integrator as far as
  the GUI is concerned?

add:
- subsurface scattering presets
- subsurface scattering builtins
- metal builtins (put spd data right in pbrt file)
- cool light spectrum builtins (ditto)
