OpenSystemsDevelopment / qpdf

Move methods responsible for loading or keeping track of objects to
QPDF_objects.cc.

Return results rather than using reference parameters.

Fixes bug in #1272 where parameters were not reinitialized when calling
read_bad_entry from read_entry.

Also, delay adjustments for compressed objects until needed by
linearization checks.

Optimistically read subsection headers without reading individual object
entries, assuming that they are 20 bytes long as per the PDF spec. If
problems are encountered, fall back to calling bad_subsections.

Replace QPDFXRefEntry with a std::variant of structs.

Temporarily disable 3 specific-bugs tests. Remove 'xref size mismatch'
test.

Split reconstruction into two passes - scanning of input for objects and
insertion of objects into the xref table. This allows insertion to take
place in the usual reverse order and removes the need for a separate
insert_reconstructed method.

Rename to Xref_table::process_section.
Process trailer processing subsection entries.

Calculate all subsections before reading subsection entries.

Duplicates some warnings for the time being.

Rename to Xref_table::subsection.
Return results instead of using out parameters.
Take on responsibility for throwing exception and calculation of offset of
first subsection entry.

Also, remove redundant clearing of the xref tabl during QPDF destruction.

Do not remove object from xref table as this is only required when the
the method is called from within QPDF::Xref_table.

To be encapsulated later.