As demonstrated in the tests, a MultiGeometry may contain other MultiGeometry nodes. We can support this with heterogenous GeometryCollection instances - though these are not currently rendered.
KML and WKT don't specify a winding order, so we write those out in CW/CCW order (for exterior/interior). GML references ISO 19107 that specifies CCW/CW, so we serialize in that winding order.
Having hand generated all this GML data the first time around, I reserve the right to modify it for the tests.
Now that we have FeatureId support (see #733), we can add this back to the
GML parsers (v2 and v3). Also add write support for FeatureId in KML, read
support was already added by @fredj
This also involves some modifications to the xmleql functionality now that
it was tested with a real-life case (KML). Also, some of the input KML files
needed to be changed since we currently cannot roundtrip 100% of the input.
