GML: don't write top <gml:boundedBy> in GML files with multiple layers of different SRS

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26131 f0d54148-0727-0410-94bb-9a71ac55c965
move member variable in right place

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26132 f0d54148-0727-0410-94bb-9a71ac55c965