fix possible regression introduced by r338

git-svn-id: http://svn.mkgmap.org.uk/splitter/trunk@340 838fabe5-b1a8-4b81-9896-7a84386980e9
improve split algo:
Increase min-nodes rapidly, reset spread and try again with smaller min-nodes threshold if no solution was found.

git-svn-id: http://svn.mkgmap.org.uk/splitter/trunk@341 838fabe5-b1a8-4b81-9896-7a84386980e9