GML: make sure that OGRGeometryFactory::forceTo() use the geometry field type and not the layer geometry type, so that works for multiple geometry field layers

git-svn-id: https://svn.osgeo.org/gdal/trunk@28006 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_vc12