Replace QPDF by PDFio as PDF manipulation library in libcupsfilters

[uddhavphatak]

As the libcupsfilters library is principally written in regular C and not C++ due to the issues which would occur due to use of C++. But libcupsfilters still depend on one library which is written in C++, QPDF, a library for manipulation of PDF files. We need to replace this dependency on C++, with another library PDFio. This is a PDF handling and manipulation library, as QPDF is, but it is fully written in standard, regular C, not in C++.

[tillkamppeter]

There is still one single C++ source file: cupsfilters/pdftoraster.cxx It is not from a QPDF dependency, it is for the use of Poppler (from the C++ library libpoppler). It can be skipped via the ./configure call with appropriate command line parameters and so we should be able to get a pure C libcupsfilters now. We will still need pdftoraster as there are some printers with buggy PostScript interpreters which do not work with Ghostscript's PostScript output (ps2write output device). A way to eliminate C++ here would be to call a command line tool of Poppler which does the PostScript rendering.

Another point is the naming of the files you have created. Could you all rename them removing the C-
in the beginning? We do not need it, as they have the .c extension and not .cxx any more.

[tillkamppeter]

By the way, your work has passed the included build and CI tests.

Could you add further test cases to the file cupsfilters/test-filter-cases.txt to exercise your changes? Note that if you need extra input files for that you need to add them to the cupsfilters source repository, and that they need to be under a free software license.

[uddhavphatak]

Sure, letme go through the poppler Library as I haven't studied anything about it. Then I would port pdftorastor.cxx into C as well. I will remove the c- suffix as well.

I will also start with test cases as well

[tillkamppeter]

Your last commit just adds the new files but named without C-. You also need to remove the C- files (actually you should git mv ... from the old name to the new name to preserve history) and you also need to update Makefile.am to use the new file names without C-.