gdalwarp: add a -ovr option to select which overview level to use, and default to AUTO. Also add a generic OVERVIEW_LEVEL=level open option, and make it available in standard VRT (#5688)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27832 f0d54148-0727-0410-94bb-9a71ac55c965
Commit updated file that should have gone with previous commit (#5688)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27833 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx