antithread: patch to antithread arabella example

I've been using the antithread arabella example to generate some
"arty" portraits for decoration.  I've made a few changes to it
(triangle sizes and number of generations before giving up), and may
send those as patches later.

Because some of the images I'm generating have taken quite a while
(many days) I've needed to restart the run after rebooting machines
for other reasons, and noticed that arabella restarted the generation
count from zero.  I wanted to continue the generation count, so here's
a patch to do just that.