OpenSystemsDevelopment / qpdf

Commit by ejb@ql.org using m-holger as author so git annotate gives
proper credit for changes.

Add methods isNameAndEquals, isDictionaryOfType, isStreamOfType

When adding a QPDFObjectHandle to an array or dictionary, if possible,
check if the new object belongs to the same QPDF. This makes it much
easier to find incorrect code than waiting for the situation to be
detected when the file is written.

Also improve behavior around direct vs. indirect resources.

Also fix a bug in checking consistency of length for stream data
providers. Length should not be checked or recorded if the provider
says it failed to generate the data.

Use the first of ASCII, PDFDocEncoding, or UTF-16 that is capable of
encoding the string.

Make some more methods in QPDFPageObjectHelper work with form
XObjects, provide forEach methods to walk through nested form
XObjects, possibly recursively. This should make it easier to work
with form XObjects from user code.

StreamDataProvider::provideStreamData now has a rich enough API for it
to effectively proxy to pipeStreamData.

Refactor removeUnreferencedResources to prepare for filtering form
XObjects.

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