Stale node deletion is implemented by maintaining a linked list
of tree nodes in vtkGeoTreeNodeCache. When a node is used, it is
propagated to the front of the list. When the list reaches a
certain size, a set of least-recently-used nodes are emptied
of their data.
BUG: Clamping bounds in the 2D projection source to eliminate
issue where image matching node bounds cannot be found in certain
cases, causing a "notch" on the edge of the earth.