GDALDataset::IRasterIO(): don't use BlockBasedRasterIO() when INTERLEAVE=PIXEL if the request band count is just 1

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26682 f0d54148-0727-0410-94bb-9a71ac55c965
VRTDataset::IRasterIO(): use source DatasetRasterIO even if band count is 1

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26683 f0d54148-0727-0410-94bb-9a71ac55c965