Patch from Jonas Koelker to improve Filling's error highlighting: as

well as marking a region as wrong if it has too many squares for the
number written in it, this patch now causes a region to be marked
wrong if it has too few squares _and no liberties_, so that it can't
just be one the user is intending to enlarge later.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9534 cda61777-01e9-0310-a592-d414129be87e