OpenSystemsDevelopment / qpdf

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.

There are lots of things with /Kids that are not pages. Repair the
pages tree, then do a reliable check.

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.