OpenSystemsDevelopment / qpdf

As discussed in #1396.

Internally use -1 to represent a missing offset and provide a constructor
overload that allows 0 as a valid offset.

In QPDF::damagedPDF use the new overload.

... containing objects with no white-space between them.

To enforce the rule that objects end at the start-offset of the next
object, each object is parsed in it own object stream.

To facilitate this, a new private API input source is::OffsetBuffer has
been added which only contains the object but reports offsets relative to
the start of the object stream. This is adapted from OffsetInputSource by
changing the direction of the offset, endowing it with its own
BufferInputSource and striooing out checks duplicated in BufferInputSource.

Fixes the expected failure in the test case added in #1266.

Add static parse methods. Make all external access to QPDFParser through
static methods.

Make all non-static methods including constructors private.

Only build strings when needed.

This was due to the use of last_object_description, which is not set for
the object stream itself.

Also, modify the messages introduced #1391 and #1392 to report the supposed
offset of the objects.

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.

This reverts commit c648b9a018105a3c30b3e7f3c5f8a058d3ddd92c, reversing
changes made to 12b67a3227df6b6df3a4f5f098e11cce173ff7d5.

This reverts commit 54cf0e519c49e3fb44c7ca1de95a1fffbafc211c, reversing
changes made to c648b9a018105a3c30b3e7f3c5f8a058d3ddd92c.

Also, make obj_cache private and rename to table.

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