OpenSystemsDevelopment / qpdf

Don't assume endobj is at the beginning of the line. This means we are
looking at tokens for every line, but the odds of n n obj appearing in
the middle of the object are likely much lower than endobj not being
at the beginning of the line or missing entirely. This will probably
have a negative impact on recovery time for very large files.
Hopefully it will be worth it.

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.

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.

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.

If we ever had an encrypted file with different filters for
attachments and either the /EmbeddedFiles name tree was deep or some
of the file specs didn't have /Type, we would have overlooked those as
attachment streams. The code now properly handles /EmbeddedFiles as a
name tree.

Refactor QPDF_Stream to use stream filter classes to handle supported
stream filters as well.

These are the comments I would have liked to have been able to read
while fixing #449 and #478.

This reverts an incorrect fix to #449 and codes it properly. The real
problem was that we were looking at the local dictionaries rather than
the foreign dictionaries when saving the foreign stream data. In the
case of direct objects, these happened to be the same, but in the case
of indirect objects, the object references could be pointing anywhere
since object numbers don't match up between the old and new files.

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.

* Return rather than exiting from realmain in qpdf.cc
* Remove extraneous blank line
* Don't assign temporary to const reference

Specifically, if a stream had its stream data replaced and had
indirect /Filter or /DecodeParms, it would result in non-silent loss
of data and/or internal error.

Fixes a crash found by fuzzing.

StreamDataProvider::provideStreamData now has a rich enough API for it
to effectively proxy to pipeStreamData.

Fixes qpdf/qpdf#419