PDFDataset lost ability to save/load GeoTransform to/from GDALPamDataset

[oleinik]

I try to load PDF without GeoTransform info inside. Such info exists in corresponding PAM-file. Call to GDALGetGeoTransform returns CE_Failure and default GeoTransform (0,1,0,0,0,1). After resolving ticket #6600 PAM-file normally loaded and can be used by PDFDataset. I think that GeoTransform from PAM can be used if such info was not found inside PDF. Or vice versa - first check for PAM and then PDF. PDFDataset::SetGeoTransform also must call GDALPamDataset::SetGeoTransform to update PAM otherwise we will lose GeoTransform (if GDALOpen used with GA_ReadOnly flag).

[Even Rouault]

In 34751:

PDF: implement reading from/writing to PAM for geotransform and projection (fixes #6603)