Fix for a mem leak in CullVisitor, the leafStack was keeping references ...

...to old nodes. I adopted the same strategy as OSG to free the leafStack. 
the point is for performance, if it does not make sense and it's faster to just re recreate from scratch then we should do that. I guess that having object already construct from frame to frame should be more efficient, and clearing as you do should be better. But that would be good to check both approach to validate it.
Because if it's not validated, I prefer to remove it and maintain less code.