Add more material types

[ianmackenzie]

Will require new shaders:

• Water
• Ceramics (very rough/retroreflective materials)
• Blinn-Phong
• Lambertian (diffuse)
• Skin (subsurface scattering)
  • Simple approximation once normal maps are supported: use normal map for specular lighting
    but not for diffuse lighting
• Thin translucent materials like leaves and lamp shades that 'glow' from transmitted light