correct handling of polygon-desc-file.

- extract parts of the polygons which are overlapped by two or
more single polygons. Split each part for itself.
- print warning when polygon is too complex instead of silently 
using the bbox

git-svn-id: http://svn.mkgmap.org.uk/splitter/trunk@336 838fabe5-b1a8-4b81-9896-7a84386980e9