The source is a total mess, mainly because it was horribly Windowsy
originally and that hasn't really been fixed yet. May even want to
reindent the source to suit me, something I'd normally be very
reluctant to do - but I've hacked this *so* much it might be
justified.

It may be possible for (completely invisible!) black Spinners to be
created on level 16 or so. Need to check if that's the case and fix if
it is. (A proper fix would be a little bit more tricky than it sounds,
though. The color changes when hit, and is set in (at least) two
different places.)