gdal_retile.py: fix to make it work with input images of different resolutions (patch by pabercrombie, #5749)

git-svn-id: https://svn.osgeo.org/gdal/trunk@28001 f0d54148-0727-0410-94bb-9a71ac55c965
gdal_retile.py: implement progress bar (patch by pabercrombie, #5750)

git-svn-id: https://svn.osgeo.org/gdal/trunk@28002 f0d54148-0727-0410-94bb-9a71ac55c965