Create a top-level branch in each tile for random objects with the

Create a top-level branch in each tile for random objects with the
name "random-objects".

Put the random objects for each tile inside a top-level
ssgRangeSelector with a maximum range of 20km.  This saves a lot of
range tests for distant tiles, and gives about a 10% framerate boost
on my card at 1000ft AGL (possibly more on faster cards).