RawRasterBand: allow direct I/O access even if a small proportion of scanlines are loaded (improve QGIS use case where the overview display will load sparse scanlines, which would prevent direct I/O at full resolution afterwards)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26899 f0d54148-0727-0410-94bb-9a71ac55c965