ece38a6dbf
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).
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