PNG: add optimized dataset RasterIO()

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@28035 f0d54148-0727-0410-94bb-9a71ac55c965
Fix typo in JPEG and PNG dataset IRasterIO

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@28036 f0d54148-0727-0410-94bb-9a71ac55c965