Immediately exits on start without error.

[rljacobson]

I'm using the Miniforge Python distribution on an M1 Mac. All dependencies appear to be available. I made a minor change to artwork.py (see #82), adding gi.require_version('GdkPixbuf', '2.0'). I get the following:

$ python3 -m photocollage.gtkgui
$

It immediately exits with exit code 0 (EXIT_SUCCESS). No window appears.