Add gallery of material effects

[ScanMountGoat]

The current documentation explains what parameters do but doesn't explain how to use them to achieve certain effects. This also should provide a better resource for people wanting to recreate an effect seen on particular mods like cel shading.

• transparency (probably best covered by updating the existing page with examples)
• metals (prm.r = 1 for non skin material)
• skin (just update the existing page with examples)
• anisotropic hair (update existing page)
• specular cube maps (sparkles, glitter, etc)
• diffuse cube maps (update page with trick for spherical meshes)
• layered textures (eyes)