drawing_ctx: move add_node_and_ancestors to rust

This might seem a little bit odd, since it adds more code than it removes, but I want to try to push toward the goals of having drawing ctx C functions called only by drawing_ctx.rs. Beside iterating the ancestors in rust is not as trivial as it sounds...

PS: If you have any suggestions on how to do it not recursively, I would very much like to know :) I tried a simple "while let .." loop, but the borrow checker defeated me