PDF: add nPixelSpace argument to ReadPixels()

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25533 f0d54148-0727-0410-94bb-9a71ac55c965
PDF: PDFDataset::IRasterIO(): add check for eBufType to trigger direct reading

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25534 f0d54148-0727-0410-94bb-9a71ac55c965