Add support for CIE standard reference light sources

[ianmackenzie]

Overcast, sunny and partly overcast, as described in http://silviojemma.com/public/papers/lighting/spherical-harmonic-lighting.pdf.

For best results they should probably use something like importance sampling taking both the BRDF and lighting equation into account (heavily weight high-reflection directions, but also the direction sunlight is coming from...some sort of product?)