OpenSystemsDevelopment / qpdf

For greater visibility and community participation, track ABI change
ideas that aren't in the source in a github discussion item rather
than in TODO.

This reverts commit 416d9668ee97c54a692d9280364746ce8aeeed43.

This commit causes numerous failures in the pikepdf test suite and
also causes a symbol to disappear from the public API. More
investigation will be required.

From time to time, someone has a problem because their build is
grabbing an old version of qpdf header files that are installed
somewhere on their system. I am hoping this will put an end to that.

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().

Removing an element from a set with iterator is constant time, and
std::set specifies that other operations on the set do not invalidate
existing iterators.

Some new symbols were exported.

These markers are being added for nested classes that are already
marked with QPDF_DLL_CLASS. They don't make any different on Linux,
but they matter on Windows.

This was broken for cross-compilation and has probably been
unnecessary for several years now.

Also fix extraneous whitespace in related some tests.

* Define POINTERHOLDER_TRANSITION
* Make sure we are getting the intended version of qpdf

Remove paragraph about traversal during destruction since this is still necessary with the
new implementation.

Replace operator== and operator!=, which were testing for the same
underlying object, with isSameObjectAs. This change was motivated by
the fact that pikepdf internally had its own operator== method for
QPDFObjectHandle that did structural comparison. I backed out qpdf's
operator== as a courtesy to pikepdf (in my own testing) but also
because I think people might naturally assume that operator== does a
structural comparison, and isSameObjectAs is clearer in its intent.

Expose functions to the C API to create new loggers and to setLogger
and getLogger for QPDF and QPDFJob.

GNU grep 3.8 started to emit warnings when invoking egrep.
Convert all calls to grep -E.

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