improve MapSplitter:
- put large objects into there own sub division
- put fewer objects into one sub division if possible so that 
more smaller divisions are created
- when splitting: don't add the bounds of the virtual
area to the bounds of the division. 

The typical effects are:
- smaller overlaps which should result in faster
redraw times on the device
- sub divisions are just as large as needed to 
hold the contained objects
- slightly larger img files
 

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3351 25d90789-57f7-4ee0-8453-03a3dfeeeb22