import_pages_with and insert_file_pages took the objects numbered above the
destination's highest number for the ones the import added. When no number
is left above the highest (an exhausted /Size, or object 4294967295),
Document::add fills free numbers below it: the copies skipped resource
sharing, and a refused import left its objects in the destination. With
object 4294967295 the `n + 1` overflowed, a panic in debug builds and in
release a wrap to 0 that counted every object as added.
Record the object numbers before the import and take the ones present
afterwards but not before.
Unhumanje
cbcdca77f0