Improve brush grid lines a bit
voxel: Change SurfaceNets mesher to support materials to a limited extent.

Right now there are just two material types that change the vertex data, geometric shape and normal type.
Very much a work in progress.
This also temporarily changes occupancy to signed distance until I figure out the right way to do this…
Add material brush and tweak generation to match signed occupancy.
gfx: Fix array texture layer uploading
voxel: Introduce simple material atlas (implemented as a texture array)
Add multiple material support on the texturing side
Add sand material (the UI code is in serious need of refactoring…)
Add more materials and fix hard-edged normals