RPC mechanism: avoid using it when creating a VRT dataset (causes a problem with gdalwarp -of VRT, since gdalwarp calls GDALInitializeWarpedVRT() on a dataset that must be a VRTWarpedDataset* and not a GDALClientDataset*) (#4979)

git-svn-id: https://svn.osgeo.org/gdal/trunk@25602 f0d54148-0727-0410-94bb-9a71ac55c965