keep board "squareness" on resize

Considere this as a partial patch: you will need to add back some UI style but also style for territory, stones, etc.

N.B: fixed an issue in `test/preview.js`, there was 19 cols/rows which were generated instead of 18 (i.e `nbrIntersection = nbrElement + 1`, so `19 = 18 + 1`)