OpenSystemsDevelopment / qpdf

For non-encrypted files, determinstic ID generation uses file contents
instead of timestamp and file name. At a small runtime cost, this
enables generation of the same /ID if the same inputs are converted in
the same way multiple times.

git-svn-id: svn+q:///qpdf/trunk@599 71b93d88-0707-0410-a8cf-f5a4172ac649