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 improves indentation of long strings. This commit also fixes some
trailing whitespace in ChangeLog.

This requires a special build option.

(So folks working offline won't go for hours dreaming they can overlay *.txt files, until after the finish writing the txt file, and try it, and it doesn't work.)

Use "QPDF" only for the QPDF class itself.

...based on a debian bug report.