General
=======

 * See whether we can do anything with /V > 3 in the encryption
   dictionary.  (V = 4 is Crypt Filters.)

 * The second xref stream for linearized files has to be padded only
   because we need file_size as computed in pass 1 to be accurate.  If
   we were not allowing writing to a pipe, we could seek back to the
   beginning and fill in the value of /L in the linearization
   dictionary as an optimization to alleviate the need for this
   padding.  Doing so would require us to pad the /L value
   individually and also to save the file descriptor and determine
   whether it's seekable.  This is probably not worth bothering with.

 * The whole xref handling code in the QPDF object allows the same
   object with more than one generation to coexist, but a lot of logic
   assumes this isn't the case.  Anything that creates mappings only
   with the object number and not the generation is this way,
   including most of the interaction between QPDFWriter and QPDF.  If
   we wanted to allow the same object with more than one generation to
   coexist, which I'm not sure is allowed, we could fix this by
   changing xref_table.  Alternatively, we could detect and disallow
   that case.  In fact, it appears that Adobe reader and other PDF
   viewing software silently ignores objects of this type, so this is
   probably not a big deal.

 * Pl_PNGFilter is only partially implemented.  If we ever decoded
   images, we'd have to finish implementing it along with the other
   filter decode parameters and types.  For just handling xref
   streams, there's really no need as it wouldn't make sense to use
   any kind of predictor other than 12 (PNG UP filter).

 * If we ever want to have check mode check the integrity of the free
   list, this can be done by looking at the code from prior to the
   object stream support of 4/5/2008.  It's in an if (0) block and
   there's a comment about it.  There's also something about it in
   qpdf.test -- search for "free table".  On the other hand, the value
   of doing this seems very low since no viewer seems to care, so it's
   probably not worth it.

 * Embedded files streams: figure out why running qpdf over the pdf
   1.7 spec results in a file that crashes acrobat reader when you try
   to save nested documents.

 * QPDFObjectHandle::getPageImages() doesn't notice images in
   inherited resource dictionaries.  See comments in that function.

Splitting by Pages
==================

Although qpdf does not currently support splitting a file into pages,
the work done for linearization covers almost all the work.  To do
page splitting.  If this functionality is needed, study
obj_user_to_objects and object_to_obj_users created in
QPDF_optimization for ideas.  It's quite possible that the information
computed by calculateLinearizationData is actually sufficient to do
page splitting in many circumstances.  That code knows which objects
are used by which pages, though it doesn't do anything page-specific
with outlines, thumbnails, page labels, or anything else.

Another approach would be to traverse only pages that are being output
taking care not to traverse into the pages tree, and then to fabricate
a new pages tree.

Either way, care must be taken to handle other things such as
outlines, page labels, thumbnails, threads, etc. in a sensible way.
This may include simply omitting information other than page content.