Support merging areas in map style

[matkoniecz]

For example, allow buildings to be merged into one object what will hide lines between touching buildings (or area:highway areas)

[matkoniecz]

http://turfjs.org/docs/#dissolve (fails for multipolygons)

[matkoniecz]

https://github.com/mfogel/polygon-clipping is the recommnded for areas

    <script src="polygon-clipping.umd.js"></script>

    <script>
      const poly1 = [[[10,0],[12,0],[10,2],[10,0]]]
      const poly2 = [[[-1,0],[1,0],[0,1],[-1,0]]]

      //alert(polygonClipping.union       (poly1, poly2 /* , poly3, ... */))
      //alert(polygonClipping.intersection(poly1, poly2 /* , poly3, ... */))
      //alert(polygonClipping.xor         (poly1, poly2 /* , poly3, ... */))
      //alert(polygonClipping.difference  (poly1, poly2 /* , poly3, ... */))
    </script>

from #22