KML: output KML that validates the ogckml22.xsd schema by placing <Schema> elements under the <Document> level. To be able to operate still as a streaming writer, the <Schema> are written at the end of the file just before the closing </Document>, which is valid (#5068)

git-svn-id: https://svn.osgeo.org/gdal/trunk@25978 f0d54148-0727-0410-94bb-9a71ac55c965