Fix geometry reassembly when we get both lines and areas for the same way
This can happen when a way-like element is also used in an area context, e.g. as part of a multipolygon relation. The Marble tile generator in this case duplicates the element, with one being clipped as a line and the other one clipped as a polygon. Attempting to merge lines and areas is futile and previously resulted in an error with most parts being discarded. Now we only discard the way-like bits, which allows us to reassemble the full geometry.