Lambros points out that trying to generate a 3x3 Cairo Easy grid

Lambros points out that trying to generate a 3x3 Cairo Easy grid
spins forever, but observes that raising the limit to 4x4 across all
grid types is not good for the complex grids like great-hexagonal.
Introduce per-grid minimum sizes using mad macro trickery.

(In fact, for each grid type I've put in a minimum size which _both_
dimensions must equal or exceed, plus another minimum size which _at
least one_ must equal or exceed; that permits both 3x4 and 4x3 Cairo
while disallowing 3x3.)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@8191 cda61777-01e9-0310-a592-d414129be87e