Calculate terrain only for bbox data

[Matthias84]

If you load an area at the sea (e.g. Rostock Warnemünde ), your dump contains the full waterways/coastlines along the land, even if it just touches your dersired area. This slows down O2W, as it tries to figure out all the area, even if this is outside the BBOX. Maybe we can add an artificial cut step, that applies the BBOX dimensions to this kind of big polys?