Support signing using PDF signatures (depends on upstream openpdf changes)

[Alkaid-Benetnash]

Currently, vector images (svg, pdf) cannot be used as signature graphic. Related issue: #183

I proposed changes to the upstream openpdf (LibrePDF/OpenPDF#1181) to support PDF as a visible signature.

This PR includes changes to jsignpdf that make use of that new feature. However, maven decides that jsignpdf should depend on a pretty old version (1.3.30) of openpdf on my end, which complicated the testing of this new feature.

I have tested in two different setups:

1. jsignpdf + openpdf 1.3.30
2. jsignpdf + openpdf master (2.0.3)

For case 1, this PR contains all jsignpdf changes. However my openpdf PR (LibrePDF/OpenPDF#1181) is not compatible with 1.3.30 (https://github.com/LibrePDF/OpenPDF/tree/1.3.30). Instead, please apply the following patch to openpdf 1.3.30:

v1.3.30-Support-using-PDF-in-addition-to-images-as-the-signa.patch.txt

For case 2, the openpdf PR contains all necessary changes. However, the current jsignpdf is not compatible with the latest openpdf. Please apply the following patch on top of this jsignpdf PR:

For-testing-purpose-adapt-jsignpdf-to-openpdf-2.0.3.patch.txt

PS: if you want to use svg signatures, it is very easy to convert them to PDF: inkscape in.svg --export-type=pdf -o out.pdf

[kwart]

Thank you for this — unfortunately this PR depends on LibrePDF/OpenPDF#1181, which was closed unmerged in May 2025. Without the upstream API this change can't compile against OpenPDF 3. Closing for now — if an equivalent API is accepted upstream in the future, please reopen a fresh PR against the current master and we'll pick it up.