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.