Support PDF version overriding from document catalog
Since PDF 1.4, PDF Version may be overriden from document catalog, something like:
1 0 obj << /Type /Catalog /Version /1.5 /Pages 2 0 R >> endobj
The PDF version as defined in the example above should be identified as a PDF version 1.5 instead of 1.4, even when the header says %PDF-1.4.
Poppler currently reports this as a PDF 1.4 document, as
PDFDoc.cc seems to set this based on header data only.