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Removed ones that were fixed upstream, and added a new one, all from the jpeg library.
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Various PDF digital signing tools do not encrypt /Contents value in signature dictionary. Adobe Acrobat Reader DC can handle a PDF with the /Contents value not encrypted. Write Contents in signature dictionary without encryption Tests ensure that string /Contents are not handled specially when not found in sig dicts.
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It seems better not to compress signature dictionaries. Various PDF digital signing tools, including Adobe Acrobat Reader DC, do not compress signature dictionaries. Table 8.93 "Entries in a signature dictionary" in PDF 1.5 reference describes that /ByteRange in the signature dictionary shall be used to describe a digest that does not include the signature value (/Contents) itself. The byte ranges cannot be determined if the dictionary is compressed.
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Table 8.93 "Entries in a signature dictionary" in PDF 1.5 reference describes that the value of Contents entry is a hexadecimal string representation when ByteRange is specified. This commit makes QPDF always uses hexadecimal strings representation instead of literal strings for it.
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Ordinarily the trailer doesn't contain any strings, so this is usually a non-issue, but if the trailer contains strings, linearizing and encrypting with object streams would include encrypted strings in the trailer, which would blow out the padding because encrypted strings are longer than their cleartext counterparts.