Fix transient_for endless loop

Other approaches would be:
- Slow/fast pointer technique.
- Using a set/associative map to save 'seen' nodes. i3 does not have
  such data structure.

Counting the total amount of windows is the simpler to implement.

I've also extracted the logic in a function and re-used it in render.c.

Fixes #4404

release-notes/bugfixes/3-transient_for

@@ -0,0 +1 @@
+Fix endless loop with transient_for windows