Add area_size() style function

area_size() calculates the area size of a polygon measured in (garmin units) ^2.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2655 25d90789-57f7-4ee0-8453-03a3dfeeeb22
area_size() returns the whole multipolygon size for mp members

In case a polygon is an outer part of a multipolygon area_size() returns the size of the complete multipolygon outer polygons.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2656 25d90789-57f7-4ee0-8453-03a3dfeeeb22