Fix for "falling through ground" problem

Fix for "falling through ground" problem

The LOD far range on the tile entry scenegraph node was initialized to
0. This meant that any traverals of active children that happened
before the tile manager updated the node would ignore the node
altogether. Among these is the groundcache traversal which was failing
at startup even though scenery was loaded.

Also added a function to dump scene graph nodes to files; very handy
in gdb.