Replace QPDF by PDFio as PDF manipulation library in libcupsfilters

This project focuses on standardization of the libcupsfilters code into C. 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++.