Warped VRT: expose as many overviews in warped dataset as there are in source dataset, and make warped VRT honour -ovr parameter of gdalwarp; change API of GDALTransformerInfo structure to introduce a pfnCreateSimilar member (#5688)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27850 f0d54148-0727-0410-94bb-9a71ac55c965