Combine sides in voxel exports

[Carifio24]

Something that we should consider to make the voxel files more lightweight is to pre-alpha-composite sides of adjacent boxes. While this won't be helpful on boundaries, this should still lead to a significant reduction in vertex and triangle count.

To be more specific, suppose we have two boxes $A$ and $B$ that are adjacent. This means that they meet along a common face. Call this face $F_A$ in A and $F_B$ in B. $F_A$ and $F_B$ are spatially coincident, but may have different opacities due to the data. What we can do is instead replace this by one common face $F_AB$ whose opacity comes from alpha-compositing together the materials of the original two faces.

This would require some modification of the geometry, but we're only dealing with rectangular prisms so this shouldn't be too difficult.

[Carifio24]

I'm actually not sure whether or not this will work due to visibility issues from various sides. Currently all of the normals for the "box" geometry point out - if we want a single side to be visible from both directions (as we would want here) then we would need to double-triangulate anyways, so maybe any gains to be had here aren't worth the effort?