tree drawing: don't pollute gDirectory, multithreading robustness enhancements
tree drawing: don't pollute gDirectory, multithreading robustness enhancements

rootpy/rootpy,kreczko/rootpy,rootpy/rootpy,kreczko/rootpy,rootpy/rootpy,ndawe/rootpy,kreczko/rootpy,ndawe/rootpy,ndawe/rootpy