Ghostscript ignores the date in the preamble and uses the current
time instead. This notably breaks the SOURCE_DATE_EPOCH support
for reproducible builds.
Passing the creation time as DOCINFO pdfmark forces gs to use the
specified date/time. Although ghostscript still adds the unreproducible
DocumentUUID and trailer ID, it is sufficient when including the PDF
figure with pdflatex.
Reuse the SOURCE_DATE_EPOCH code from creation_date for determining
the wanted timestamp, and return the formatted time via the new
`creation_date_pdfmark` function.
Stefan Brüns
2023-03-09
Thomas Loimer
2023-03-12
