OpenSystemsDevelopment / qpdf

Also, reject them in strings.

Also. remove '.' as  starting char in lsTop.

When the schema wants a variable-length array, allow a single item as
well as allowing an array.

Call the parent container's item method before calling the child
item's start method so we can easily know the current nesting level
when nested items are added.

Bidirectional base64; will be used by JSON v2.

The PDF spec only allows UTF-16BE, but most readers seem to accept
UTF-16LE as well, so now qpdf does too.

There are codepoints in PDFDoc that are not valid UTF-8 but map to
valid UTF-8. We were handling those correctly with bidirectional
mapping.

However, if those same code points appeared in UTF-8, where they have
no meaning, they were left as fixed points when converting to PDFDoc,
where they do have meaning. This change recognizes them as errors.