Rendering order for 2D shapes

[ErikKoinberg]

I am using 2d objects with a 2d camera. I am wondering how to select what objects are rendered on top. Currently I have an iterator of Gm:s that I render, but the last entry of the iterator does not always end up on top, which I had expected. It seems to depend on where the objects are, so maybe it has to do with the distance to the camera. Anyway, is there a z-index or some other simple fix to this?

[asny]

Please see #446 (comment)