OpenSystemsDevelopment / qpdf

Enhance error checking of object stream object ids and offsets

The original test file contains multiple entries with id 0 and offset 0.
One entry has been modified such that the id is valid (6).

Object streams with invalid offsets are a source of unreproduceable
oss-fuzz time-outs.

and delay to qpdf 13 to give users notice of the change.

Push mode is not used by qpdf and is not tested. It is difficult to see a
legitimate use for it.

Continuing to support it creates unnecessary constraints and effort when
adapting the tokenizer.

If duplicate keys are encountered, overwrite earlier values with the latest
value.

Also, move Buffer::Members definition to Buffer.cc

This requires a special build option.

Use "QPDF" only for the QPDF class itself.

Holding release to think about recently discovered incorrect handling
of deleted generations.

This reverts commit 162716807c475498dfbecc89fd222fb6c45d70c9.

...when flattening annotations. This includes /Link, /Popup,
and /Projection.

Rather than adding yet another flag that is almost never useful on its
own, just have --remove-restrictions disable digital signatures. The
operations are still separate in the API.