Quadtree LOD

Implemented a Quadtree for mesh LOD. Subdivides based on apparent size
of each chunk.

TODO:

*Heightmaps and seam repair.
*Less expensive math for LOD. Maybe lookup tables?
*Less clunkiness in general.
*Factor flatness into LOD calculation. (After heightmaps.) Also, factor
in angle of surface to camera. Not a lot of detail is required when
looking straight down at a mountain. Normal maps will do most of the
work there. Conversely, a ton of detail is required for stuff against
the horizon.
*Culling, of the occlusion and frustum varieties. Stuff behind the
planet or behind the camera shouldn't be drawn.