Lots of improvements from review, etc.

- In the QuadtreePrimitive Sandcastle example, compute bounding spheres for
the rectangles rather than using the unreliable-in-2D RectangleGeometry
ones.
- Add Camera.positionCartographic property.
- Disable depth testing in 2D.  It was accidentally enabled during this
  refactoring.
- Don't pass camera positions in to getDistanceToTile.