OpenSystemsDevelopment / qpdf

Support conversion of pages to form XObjects and placement of form
XObjects on pages.

Add getAttribute for handling inheritable page attributes, and fix
getPageImages and annotation flattening code to use it.

Explicitly abandon removal of unreferenced resources if there are any
lexical errors in the page's contents. This case always generated a
warning, but it now also prevents removal of unreferenced resources,
this strongly decreasing the likelihood of data loss.

When removing unreferenced resources, the code was copying the overall
resource dictionaries but not the subdictionaries being modified. This
was a "typo" in the code -- the comment clearly stated the need to do
this, but the code replaced the dictionary with itself rather than
with a shallow copy of itself.

If we are unable to filter a page's content streams, don't attempt to
remove objects from the page's resource dictionary. Also provide a
command line option to suppress resource removal in case we ever need
this as a workaround for some bug or broken PDF files.

This is the beginning of higher-level API support using helper
classes. The goal is to be able to add more helpers without continuing
to pollute QPDF's and QPDFObjectHandle's public interfaces.