Fixes http://www.pdf-archive.com/2013/09/30/file2/file2.pdf.
Note how it's not possible to show the various Popup Annotations in the above document.
To fix that, this patch lets the Popup inherit the flags of the parent, in the special case where the parent is `viewable` *and* the Popup is not.
In general, I don't think that a Popup must have the same flags set as the parent. However, it seems very strange to have a `viewable` parent annotation, and then not being able to view the Popup.
Annoyingly the PDF specification doesn't, as far as I can find, mention anything about how this case should be handled, but this patch seem consistent with the actual behaviour in Adobe Reader.
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