Generate PBR maps

Re: #751 - One of the issues blocking PR #1276 from being merged is what to do in the event a certain block surface doesn't have a PBR map. Or, in the event that no PBR resource pack is loaded can we still somehow implement PBR features.

Generating PBR maps may prove useful.

- Emittance Maps - Multiply the emittance by the perceived brightness of the albedo and have a customisable threshold slider.
- Normal Maps - Treat texture as "grayscale" heightmap. ie if the pixel is brighter than the one to its left the normal will point to the left (same thing for up down and right). FYI - Complementary Shaders use the length function = sqrt(R² + G² + B²) so it's not exactly grayscale.

PBR maps that it may not be possible to generate:
- Perceptual Smoothness
- Reflectance (F0)/Metalness
- Porosity/Subsurface scattering
- Material AO
- Height Map