Recursive glyph references choke glyphsLib

(Filing this here to take it out of my inbox)

Jany found a file with the following properties:

* parenleft is a glyph with outlines, and in the background it has bracketleft as a component
* bracketleft is a glyph with outlines, and in the background it has parenleft as a component

> The situation is valid but glyphsLib chokes on it (`RecursionError: maximum recursion depth exceeded while calling a python object.`) because for some reason it creates a component loop in the same layer... to investigate further