Implement loading documents from FD

Add loading a document from a file descriptor.

This is the basis for some previewer work to replace --unlink-tempfile with the ability to pass an already-unlinked (possibly O_TMPFILE) temp file.

The patch series implements the new document interface for poppler (depending on https://gitlab.freedesktop.org/poppler/poppler/-/merge_requests/945), ps (not ready in libspectre yet) and tiff. I also have experimental (not yet working fully) code for xps, and comics backend should be easy to do, too. However, for the previewer, really only pdf needs to work.