correction part 3: relations may be missing when split-file is given
that contains overlapping tiles. The error was introduced with r316
and only partly corrected with r327 and r330.

The generation of non-overlapping areas did not work if
two sets of tiles are covering the same area.
As a result of this fix you may see increased run time
in case your split-file contains areas with are covered 
by more than two different tiles.

git-svn-id: http://svn.mkgmap.org.uk/splitter/trunk@331 838fabe5-b1a8-4b81-9896-7a84386980e9