OpenSystemsDevelopment / qpdf

This covers up forgetting to use QPDF_DLL and QPDF_DLL_CLASS.

Just because we know an indirect reference is null, doesn't mean we
shouldn't keep it indirect.

For some reason, qpdf from the beginning was replacing indirect
references to null with literal null in arrays even after removing the
old behavior of flattening scalar references. This seems like a bad
idea.

This message used to only appear for PDF >= 1.2. The invalid name is
valid for PDF 1.0 and 1.1. However, since QPDFWriter may write a newer
version, it's better to detect and warn in all cases. Therefore make
the warning more informative.

This change works around STL problems with Embarcadero C++ Builder
version 10.2, but std::vector is more common than std::list in qpdf,
and this is a relatively new API, so an API change is tolerable.

Thanks to Thorsten Schöning <6223655+ams-tschoening@users.noreply.github.com>
for the fix.

There were a few cases that could be caused by invalid input rather
than bugs in the code which were throwing logic_error instead of
runtime_error.

This also reverts the addition of a new checkLinearization that
distinguishes errors from warnings. There's no practical distinction
between what was considered an error and what was considered a
warning.

Thanks to github user zdenop for supplying some additional
error-handling code.

Use PointerHolder in several places where manually memory allocation
and deallocation were being used. This helps to protect against memory
leaks when exceptions are thrown in surprising places.