Speedup is non-linear, 2x-10x faster is quite normal.
Patch T43678.
- Switched from an Octree to BVH.
- Finding first points of surface no longer "wastes" density function evaluation: every result is cached.
- Use MemArena instead of using own memory management.
- Correct calculation of metaelem bounding box.
- Remove mball_count(): mballs are now counted "on the go".
