Fix growing memory consumption issues

Fix growing memory consumption issues
References in simgear's cache maps prevented effects, textures, clouds,
trees and lights from being unloaded at run-time (ref-counter could never
reach 0). Changed cache maps to use OSG 'observer' pointers instead, so
reference counters aren't influenced, though cache maps still hold an
(observing) pointer. Also take care when finding a cache entry with empty
content (i.e. texture was unloaded).