OpenSystemsDevelopment / qpdf

This reverts commit bb045907a043b5c6de9fb804ff11087333747329.

Split QPDF.cc into QPDF.cc and QPDF_objects.cc

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

The split was part of the reverted #1297. Reintroducing it now makes it
easier/safer to keep the work to refactor the xref and object tables in
sync with main.

Refine recovery from missing startxref (fixes #1335)

Refactor xref table reconstruction (Fixes #1362)

Refactor InputSource

linearization parameter dictionary.

Spec states that that linearization parameter dictionary must be the first
object.

Improve support for reading into std::string objects.

Move qpdf private inline methods to private header file.

Minor code tidy and update .git-blame-ignore-revs

Ignore reverted #1272, #1289, #1297 and #1301.
Ignore last two commits.

If startxref cannot be found in the last 1024 try finding it in the
whole file and check whether it is valid.

Split reconstruction into three passes - scanning of input for objects and
trailer, insertion of objects into the xref table, and loading the trailer.

This allows insertion to take place in the usual reverse order and removes
the need for a separate insertReconstructedXrefEntry method.

It also allows trailer to be tried from most recent to oldest.

Ignore any found trailers without /Root entry.

I always think of doing this after I have already tagged the release.

There simply is no chance Members will be required during the life of
qpdf 12.

Figuring out which classes are part of the public API by using library
symbols is fragile (dependent on specific compiler optimizations) and
unreliable (misses some inline things). Instead, use castxml, a tool
that parses C++ to an abstract syntax tree and generates XML, to get a
reliable accounting of public classes and their sizes.

...and include ObjectHandle.hh first in QPDFObjectHandle so there is
at least one place (other than check_abi get-sizes) that includes it
first.

Use BaseHandle::operator QPDFObjGen in examples

Fix two BaseHandle bugs

- Add missing export for operator QPDFObjGen.

- Fix handling of indirect references. The use of the type_code method
  in various switch statements bypassed the special handling of
  ::ot_reference objects. Replace with new method resolved_type_code.

Add tests for ::ot_reference objects.

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.