performance improvement: use Path2D.append() followed by one 
new Area(path) instead of multiple Area.add()
Reduces run time for the generation of precompiled sea data by ~70%
without notable changes in memory usage or output


git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2982 25d90789-57f7-4ee0-8453-03a3dfeeeb22