LIBKML: when reading a gx:Track set the first timestamp as 'begin' and the last one as 'end', if not already set

git-svn-id: https://svn.osgeo.org/gdal/trunk@27112 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx