Using forked pdfcpu

[agcom]

Currently, instead of the actual upstream pdfcpu, a forked version is used (https://github.com/agcom/pdfcpu) to solve some shortcomings in extending the library. See pdfcpu/pdfcpu#1145 and pdfcpu/pdfcpu#168 (comment).

Maybe find a workaround to use the upstream pdfcpu directly, or convince the maintainer to upstream the fork changes.