I've proved that a grid dimension of 2 is capable of irretrievably

hanging the grid generator and that there's no way to fix it.
Therefore, lower limit of 3 squares in each direction, which will
upset envelope-pushers everywhere but doesn't destroy any actually
interesting puzzles.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5895 cda61777-01e9-0310-a592-d414129be87e