1) Remove dependency on the /sim/rendering/shader-effects
2) Fix http://code.google.com/p/flightgear-bugs/issues/detail?id=494 where
toggling the random vegetation on, iff initiall set off, did not generate
new random vegetation. Fixed by forcing a tile reload in the same way as
used for random objects.
Draw trees in two passes, opaque and transparent. This allows us to
have some transparency, which is better looking, and dispense with
sorting.
Author: Tim Moore <timoore@redhat.com>
