OpenSystemsDevelopment / qpdf

Explicitly abandon removal of unreferenced resources if there are any
lexical errors in the page's contents. This case always generated a
warning, but it now also prevents removal of unreferenced resources,
this strongly decreasing the likelihood of data loss.

The tests are in a separate commit so the bug-fix commit can be taken
as a patch for older versions.

When removing unreferenced resources, the code was copying the overall
resource dictionaries but not the subdictionaries being modified. This
was a "typo" in the code -- the comment clearly stated the need to do
this, but the code replaced the dictionary with itself rather than
with a shallow copy of itself.

If set, we avoid using Windows I/O HANDLE, which is disallowed in some
versions of the Windows SDK, such as for Windows phones.
QUtil::same_file will always return false in this case. Only applies
to Windows builds.

The original QPDF is only required now when the source
QPDFObjectHandle is a stream that gets its stream data from a
QPDFObjectHandle::StreamDataProvider.

This is in preparation of being able to pipe a stream's data without
keeping a copy of its containing qpdf object.

Use this instead of QPDF* as a map key for object_copiers.