OpenSystemsDevelopment / qpdf

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.

Also, when recovering trailer from xref streams, pick the last valid
trailer encountered rather than the first.

Fix handling of hybrid reference files in QPDF::read_xrefTable

Fix QPDF::recoverStreamLength

Tidy QPDF_Stream

1. Make class final
2. Pass og parameter by value
3. Properly initialize qpdf and og

Also, tweak QPDF::replaceObject to allow stream replacement without
violating the requirement that streams must always be indirect objects.

Also, remove QPDF::reserveStream as it does not do what the name implies
and having this as a separate methods does not aid code readability.

Ensure the the recovered stream end is not part of a different object.

Test file is bad24.pdf with stream 4 'endstream' corrupted.

QPDF::read_xrefTable ignores type 0 entries for objects in a section if an
associates XRefStm has an entry for the same object.

The spec states:

When the conforming reader searches for an object, if an entry is not
found in any given standard cross-reference section, the search shall
proceed to a cross-reference stream specified by the XRefStm entry
before looking in the previous cross-reference section,

If a deleted entry is found in a section, the XRefStm is not searched
according to the standard.