Modifed random-object code for more memory efficiency:

Modifed random-object code for more memory efficiency:

1. Only one copy of each object (and its texture) is stored in memory,
   no matter how many materials use it.

2. Random objects are added to a triangle only when the objects are in
   range, and they are deleted as soon as the objects are out of
   range.  That way, only a relatively small number of ssg nodes are
   used at any one time.

This patch seems to eliminate freezes when tiles are freed.  There are
occasional stutters at extremely high speeds (i.e. over 3,000kt), but
it seems smooth enough for normal aviation speeds.