OpenSystemsDevelopment / qpdf

An indirect object reference to 0, 0 is invalid. If it appears in the
file or is parsed from a string, the parser catches it. This check
would only be useful for someone explicitly calling getObject with 0,
0, and that would trigger an error during resolve().

QPDFValueProxy wasn't a good name for it. We decided the evil of
having the header file be named QPDFObject_private.hh was less than
the evil of having the class be named something other than what it
should have been named.

Preparing to change the class name back to QPDFObject

I decided that it's actually fine to copy a direct object to another
QPDF. Even if we eventually prevent a QPDFObject from having multiple
parents, this could happen if an object is moved.

When a QPDF is destroyed, changing indirect objects to direct nulls
makes them effectively disappear silently when they sneak into other
places. Instead, we should treat this as an error. Adding a destroyed
object type makes this possible.

This is in preparation for restoring a QPDFObject.hh to ease the
transition on qpdf_object_type_e.

This commit was created by
* Renaming QPDFObject.cc and QPDFObject.hh
* Replacing QPDFObject\b with QPDFValueProxy (where \b is word
  boundary)
* Running format-code
* Manually resorting files in libqpdf/CMakeLists.txt
* Manually refilling the comment in QPDF.hh near class Resolver

On destruction of the QPDF object replace all indirect object references
with direct nulls.

Remove all existing code to release resolved references.

Fixes performance issue due to interaction of resetting QPDFValue::qpdf and
og members and prior code.

Now that QPDFObject.hh is out of the public API, we don't really need
QPDFObjAccessor. Also, the function didn't have to be virtual anymore.

This required moving some newly inlined functions back to the cc file,
but that seems to have had no measurable performance impact.

Also, modify QPDFObject::swapWith to update the ObjGens of the swapped
objects.

Modify QPDF::newIndirect and QPDF::updateCache to keep object ObjGens
up to date.

Add a new obj_cache entry or update an existing entry in place.

Also change QPDF::replaceObject and QPDF::swapObjects such that the
QPDFObject assigned to an og in the obj_cache is never replaced; only
QPDFObject::value is updated.

Centralise casting of QPDFObjects and reduce repeated dereferencing.

Also, inline method and add optional parameter error_msg.

Code tidy: remove redundant calls to smart_ptrs get() method

For consistency with similar methods, e.g. replaceObject.