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)

[Metadorius]

Would make sense to have three different variations:

• unlit colors only, no shading
• no colors, only shading according to normals (think of how untextured models look)
• both colors and shading according to normals

[Metadorius]

For an editor I don't think it's necessary to replicate VPL mechanism exactly, can just use the palette color as an unlit color and then apply your own shading instead of VPL, the point is just to treat the each individual voxel as having one normal vector per all the sides instead of 6 normals orthogonal to sides.