data loss splitting pages with indirect XObject or Font

[jberkenbilt]

This bug was reported to me by email. I am including it here for the record. This is the first silent data loss bug ever found in qpdf. When splitting/merging pages and discarding unreferenced resources, if the /Font or /XObject keys in the page's resource dictionaries were indirect and some XObjects or Fonts were referenced on some but not all of the pages, only images or fonts referenced on all pages would be maintained. The root cause was basically a typo. The comment in the code explained clearly how to handle this case, but the code was actually replacing the dictionaries with themselves rather than with shallow copies of themselves. The fix is a single line of code. The commit that fixes this applies cleanly to all released versions of qpdf from 8.1.0 forward. The bug was introduced in version 8.1.0.

[jberkenbilt]

Note that users can run with the --preserve-unreferenced-resources flag to work around the problem.