Volumetric terrain

[bfops]

Store/generate/modify terrain with volumetric data structures (something voxel-ish).

• Sparse Voxel Octree (86962f8)
  • 1/2^n width voxels (1b0f866).
• Isosurface extraction (generate a mesh from the SVO) (86962f8).
  • Extract normals (eb90943).
• Collision detection between Aabb3s and SVO (#120).
• Voxels of different materials (#121).
  • Voxel trees (#122).
• Represent the same space at several LODs (relevant: bfops/bit-svo#2).
• Voxel terrain generation (generate SVO data from the heightmap) (2a3c4f9).
• Terrain texturing (this might be useful) (20994f2).

[bfops]

Based on this article and some thinking, I'm thinking we treat everything as a contiguous volume. We store a sparse voxel octree, where each cell is either entirely inside or entirely outside the volume, or it contains a single surface vertex and a record of where the surface intersects the edges. If the surface intersects an edge, all the cubes adjacent to that edge (four of them) intersect the surface, too, so we generate four triangles using the four cubes' vertices and the intersection point of the edge.

[bfops]

WIP on the voxel branch.

[bfops]

Moving the pre-volumetric terrain to the pre-svo branch, merging the voxel branch into master, and moving leftover list items into separate issues.