OpenSystemsDevelopment / qpdf

Add optional parameter allow_nullptr to QPDFObjectHandle::getOwningQPDF

Also, inline method and add optional parameter error_msg.

Provide buffered input for QPDFTokenizer.

Avoid building error message each call "just in case".

This results in measurable performance improvements to packaged binary
libqpdf distributions. QTC remains available for library users and is
still selectively enabled in CI.

It turns out that QUtil::get_env is particularly expensive on Windows
if there is a large environment. This may be true on other platforms
as well.

This is a guess...I'm not sure exactly why there are linker issues or
how to reproduce them.

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