GML TopoSurface: fix duplicated points in GML_FACE_HOLE_NEGATIVE=YES mode (patch by Sandro Furieri, #5230)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26559 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx