Improve the algorithm for figuring out where the number should be

Improve the algorithm for figuring out where the number should be
drawn in a face: averaging the vertex positions works fine for regular
or roughly regular convex polygons, but it'll start being a pain for
odder or concave ones.

This is a kludgey brute-force algorithm; I have ideas about more
elegant ways of doing this job, but they're more fiddly, so I thought
I'd start with something that basically worked.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9137 cda61777-01e9-0310-a592-d414129be87e