Python bindings: Band.ReadRaster() and Dataset.ReadRaster(): clear the buffer in case there are holes in it due to odd spacings specified by the user

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27807 f0d54148-0727-0410-94bb-9a71ac55c965
GDAL Mem: implement optimized versions of raster band and dataset IRasterIO()

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27808 f0d54148-0727-0410-94bb-9a71ac55c965