Add GDALMemDataset example.

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25794 f0d54148-0727-0410-94bb-9a71ac55c965
Fix a few compiler warnings

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25795 f0d54148-0727-0410-94bb-9a71ac55c965