Voxel light-emission rendering about all-is-cubes

[kpreid]

Preferably without allocating twice the texture space, but that would require being able to split meshes by material (which would be useful, but hairy).

From discussion with Ralith on the Rust GameDev Discord server: Note that we don't have to entirely split meshes and use separate draw calls. We can just have multiple bound textures, and store in the vertices an index of which texture set, if any, to use, like so:

• 0 = use vertex color
• 1 = use (reflectance atlas 0, constant emission zero)
• 2 = use (reflectance atlas 1, emission atlas 1)

My other idea prior to this was to indirect through a table of texture coordinate offsets, so that the emissive data could be written to the same texture in a different location; that would have the advantage of fewer textures and supporting things like defragmenting the texture atlas, but is likely to cost more.

[kpreid]

As of commit 2e9f6c6 we have a plan for how to allocate textures as above, by having the mesher tell the allocator what Channels are needed.

Next steps:

• Have the allocators actually allocate separate textures (and designate them in the vertex coordinates)
• Implement emission in the shaders
• Have the mesh generation switch to textures even for simple blocks if there is an emission value (this way, we don't need two vertex colors).

[kpreid]

As of commit a2fd653, emission rendering is active. It is not yet implemented in the glTF export, but since texturing is still generally a work in progress there, I'm going to call this done.