Implement XRefStm handling in PDFUtilFnc

[erikn69]

Added handling for XRefStm in PDF trailer processing.
object-streams.html#cross-reference-streams

trailer
<</Size 23/Root 1 0 R/Info 9 0 R/ID[<CC624E32F9811843A1DC762908A2F2B3><CC624E32F9811843A1DC762908A2F2B3>] >>
startxref
27721
%%EOF
xref
0 0
trailer
<</Size 23/Root 1 0 R/Info 9 0 R
/ID[<CC624E32F9811843A1DC762908A2F2B3><CC624E32F9811843A1DC762908A2F2B3>] 
/Prev 27721/XRefStm 27438>>
startxref
28337
%%EOF

I prefer that they check it thoroughly before merging; it works for me, but I don't know if it's the best approach.
I had to send it to production and it works for me