OpenSystemsDevelopment / qpdf

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

Add QPDF::getObject to replace getObjectByObjGen and getObjectByID

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

Also, make QPDFObjectHandle::isIndirect const.