Flatten annotations by integrating their appearance streams into the
content stream of the containing page. In the case of form fields,
only flatten if /NeedAppearance is false (or equivalently absent). If
flattening form fields, also remove /AcroForm from the document
catalog.
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.