PBR textures support (Normal maps, height maps, emitter maps, specular maps)

[carterlasalle]

Allows support for PBR texture packs, so 3d textures get rendered

[leMaik]

This is a big feature and actually PBR includes multiple features:

• [x] Normal maps (implementation started)
• [ ] Height/Parallax Occlusion maps
• [x] Specular maps
  • [x] Emitter maps
  • [x] Roughness maps
  • [x] Reflectance maps
  • [x] Metalness maps
    • [ ] Predefined f0 values for complex Fresnel equation (LabPBR 1.3)

Branch with normal maps: https://github.com/leMaik/chunky/tree/json-block-models-and-normal-maps (it's pretty slow. I hope that it'll be ready to be merged some day though)

Branch with specular maps (emittance, roughness, reflectance): https://github.com/leMaik/chunky/tree/pbr

Height maps are a whole different topic, but @jackjt8 and me already looked a bit into how it can be done.

Same issue in my fork: https://github.com/leMaik/chunky/issues/14