OpenSystemsDevelopment / qpdf

Centralise casting of QPDFObjects and reduce repeated dereferencing.

Part of #729

Part of #729

Part of #729

Also, inline method and add optional parameter error_msg.

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

Also, make QPDFObjectHandle::isIndirect const.

Code tidy : replace 0 with nullptr or true

Remove any ambiguity around whether old or new value is being
returned.

Use 'QPDFObjGen const&' instead of 'int, int' in signature.

Change method signatures to use QPDFObjGen.
Replace QPDF_Stream::objid and generation with QPDF_Stream::og.

Change method signatures to use QPDFObjGen.
Use QPDFObjGen methods where possible.
Remove redundant QPDF::objGenToIndirect.

Change method signature to use QPDFObjGen.

Also, revert inlining of unparse and operator << from commit 4c6640c in
order to avoid exposing QUtil.

Move responsibility for creating shared pointers to objects and cloning from QPDFObjectHandle to QPDFObject.

Also, use methods to access objid and generation.

When there is no context for writing output or error messages, use the
default logger.

Remove variables obsoleted by commit 4f24617.

I decided these were confusing and inconsistent with how JSON works.
They muddle the API rather than improving it.

When an empty string was passed to replaceStreamData, the code was
passing a null pointer to memcpy. Since a 0 size was also passed, this
was harmless, but it triggers sanitizer errors. The code properly
handles a null pointer as the buffer in other places.

Not fully exercised in this commit

Update getJSON() methods and calls to them

This reverts commit dc059560e73e0b373a6e54e71b07e3af4b692cb4.

I changed my mind. There's no harm in leaving it deprecated for a
release cycle.

See ChangeLog for rationale for not deprecating it as originally
planned.