mixed (ref-less and ref-full) out geom content leads to broken multipolygon rendering

[tyrasd]

See tyrasd/overpass-turbo#182

another test case: http://overpass-turbo.eu/s/apQ

[tyrasd]

this is potentially related to #23

[tyrasd]

The problem is that we have one part of the multipolygon's ways with node refs, while the rest is provided ref-less (just node coordinates, from the output of out geom on relations). This confuses the multipolygon assembly algorithm which relies on either all nodes having proper ids or all nodes being ref-less.

This is going to be quite tricky to solve. One possibility would be to greatly improve #23 together with a more clever multipolygon assembly. Another way would be to parse each full-geometry element independently from each the rest of the supplied content, which sounds like a reasonable thing to do.