Explicitly support 3D texturing in the texture allocator

[kpreid]

Right now, all-is-cubes-mesh’s generation of texture coordinates for volumetric rendering is cheating by extending past the range a texture::Plane covers. To fix this, we should add explicit optional support for 3D texturing to the texture allocator traits’ contracts. In particular, it would probably be simplest to change the rule of texture::Tile::slice() that the slice must have a size of 1 on some axis, so that instead it will optionally succeed with a 3D volume.