only draw contour shade for real osm polygons

[hholzgra]

Only draw contour shade for real osm polygons, not for simple rectangular bounding box

When just selecting a bounding box in the web interface the exact area is not really important, and as the aspect ratio of the selection will usually not match that of the map canvas rendering area this leeds to confusing dark strips at the top and bottom, or left and right, of the rendered map, depending on which direction the selecting bounding box is extended to to fill the render canvas