OpenSystemsDevelopment / qpdf

Also improve behavior around direct vs. indirect resources.

Converted ResourceFinder to ParserCallbacks so we can better detect
the name that precedes various operators and use the operators to sort
the names into resource types. This enables us to be smarter about
detecting unreferenced resources in pages and also sets the stage for
reconciling differences in /DR across documents.

If not found in the field hierarchy, /Q and /DA are supposed to be
looked up in the document-level form dictionary. /DR is supposed to
only come from the document dictionary.

This results in a performance penalty of 1% to 2% when replaceObject
and swapObjects are never called and a somewhat larger penalty if they
are called, but it's worth it to avoid very confusing behavior as
discussed in depth in qpdf#507.

This takes pages from the file in groups of n with default = 1. This
partially fixes the enhancement in issue #505 but doesn't implement
the entire suggestion.

Also fix a bug in checking consistency of length for stream data
providers. Length should not be checked or recorded if the provider
says it failed to generate the data.

Use C++14 [[deprecated]] tag

I thought /EFF was supposed to be used as a default for decrypting
embedded file streams, but actually it's supposed to be advice to a
conforming writer about handling new ones. This makes sense since the
findAttachmentStreams code, which is not actually needed, was never
right.

For now, --allow-insecure allows creation of files with the owner
passwords empty or matching the user password.

When removing unreferenced resources, notice if a page (recursively)
contains a form XObject with unreferenced resources, and count any
such resources as referenced by the page.

Warnings were not being handled per --no-warn or generating exit code 3.

Keep a std::pair internal to the iterators so that operator* can
return a reference and operator-> can work, and each can work without
copying pairs of objects around.