- use MeshLambertMaterial for interiors, terrain, and shapes
- use smooth vertex normal blending to avoid facted-looking contrasty lighting
between adjacent surfaces
- update io_dif Blender addon to extract lightmaps
- re-export .dif files to glTF with lightmaps and without LOD
- enable sun, ensure correct direction
- adjust fog (more work to do)
- cleanup and optimization