Fix center point calculation of large ways and polygons

There was a possible integer overflow for ways and polygons with a large number of points or in case the points were located far aways from 0°/0°. In this case the center point calculation returned a wrong location.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2566 25d90789-57f7-4ee0-8453-03a3dfeeeb22
roads_first_v1.patch: Make sure that roads are placed before other lines
in each sub division.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2567 25d90789-57f7-4ee0-8453-03a3dfeeeb22