[Therion] Infinite loop in therion 5.3.9

[Bruce]

> It is because I had no walls, and had hidden the centreline
symbol group. So there is nothing to draw on the PDF and this causes
the issue. 

Dang! I thought of this possibility after my last post.  I have even done it
myself a few times.
Lesson for the day.  When debugging problems with 2-d projections, first
disable the layouts as they often contain the problem entries!

Bruce