Efficiency / tile (un)loading

[iannesbitt]

• [ ] Matt noted that the viewer bogs down after a few minutes of browsing
  • [ ] Tiles do not unload/faraway datasets cause further performance issues
  • [ ] Screen space error seems to be improperly set
• [ ] Tile loading is inconsistent with the camera location (I think it's partially related to the camera, but mostly to the focus point)
• [ ] Merge function bounding box calcs/view errors (#9)