Allow a 1-omino to be completely destroyed and recreated in an

Allow a 1-omino to be completely destroyed and recreated in an
arbitrary unclaimed square. This cures the most common cause of
generation failures (covering a large area in dominoes was the most
difficult case, and would fail even if the large area was 1xn!); the
failure rate is now sufficiently low under all circumstances I've
found that I'm willing to just loop until I get a success.

[originally from svn r7693]