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. 
I've seen till 200 leafs more or less. I see the benefits to have a renderLeafStack, at least in C++ you don't go through constructors again, I guess for JS is the same. In fact I'm only freeing from the begining of the stack to the current index. So it always should be synced and no references to leafs should be keeped.