Culling / OBB optimizations

[pjcozzi]

Ideas for later.

• Use OBB for determining what frustums a command belongs in. This should result in fewer commands overlapping more than one frustum. It will have additional overhead, but perhaps we can exploit temporal coherence to reduce the number of distance checks?
• Instead of culling terrain/buildings/etc with the tile's OBB, use the union of the children's OBBs. It will be more precise but will have more operations. Perhaps use spatial coherence to minimize the number of checks. 3D Tiles now does this, Add support to check children bounds for visibility before selecting a tile #4978

References

• http://cesiumjs.org/2015/06/24/Oriented-Bounding-Boxes/
• https://fgiesen.wordpress.com/2010/10/17/view-frustum-culling/
• http://www.cse.chalmers.se/~uffe/vfc_bbox.pdf
• http://www.cse.chalmers.se/edu/year/2009/course/TDA361/2006/TDA360/Designing%20a%20PC%20Game%20Engine.pdf
• https://smartech.gatech.edu/bitstream/handle/1853/3486/98-38.pdf (BV ideas)