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.

In FUTURE, treat uninitialized object handles as null

Fixes fuzz issue 71689.

Change the return type to a std::pair<bool, QPDFObjectHandle> in order to
allow a default constructed object handle (which is currently returned to
indicate failure) to become a valid object.

Fix handling of hybrid reference files in QPDF::read_xrefTable

Fix #1242

Fix QPDF::recoverStreamLength

Tidy QPDF_Stream

Replace some std::shared_ptr parameters with reference parameters

Ensure QPDF m->all_pages and invalid_page_found are reset if
getAllPagesInternal throws an exception.

Fixes fuzz case 71624.