Skip root node in RenderTree iterator

[Kiyoshika]

          Although now that I think about it, for the purpose of the `RenderTree`, the root node is technically not a visible node so should probably be skipped in the iterator itself

Originally posted by @Kiyoshika in #293 (comment)

[Sharktheone]

Makes sense to me. Else you'd have to always manually check if you are getting the root node. I think, when we need this, we could add another function like RenderTree::next_root.