VOXEDIT: Add flat/normal-based/C&C voxel shading mode

[Metadorius]

Basically in this mode each cube is rendered flat without any kind of shading if there's no normal information, and the color is picked as if it was a plane with a normal corresponding to the normal vector that this specific cube has.

This is how the game is drawing the voxels more or less, basically the game has material parameters for a basic shading model baked in as VPL - basically it defines 256 colors * 32 light levels - which it uses to shade the voxel (screenshot from https://github.com/ThomasSneddon/vxl-renderer, in order to use you have to drop a vxl file onto the .exe though)