Fix voxelized sponza

[Robert42]

• [x] allow joining the whole sponza building mesh (ceiling, floors, walls, archs, no columns!) into a single voxelgrid
  • [ ] Be aware of meshes, which are not closed. For example the floor of the second floor is single sided with nothing below it within the same mesh
  • [x] allow merging them? (for example all static meshes of the building with the same size) => improved performance, less artifacts, less memory
  • [ ] add a debug shader allowing to highlight parts, where the negative area of a distancefield is touched
  • [ ] add geometry to close holes?

[Robert42]

a5999043