Here's the new version of the patch, which enables using MuPDF to open
password-protected PDF files.

One problem I encountered while writing it is that function
`doc-view-pdf->png-converter-mupdf' uses a small hack to add "draw" to
the arguments list passed to `doc-view-start-process', only when
`doc-view-pdfdraw-program' has the value "mutool". This is because the
"mudraw" command has been replaced at some point by the "mutool"
command, which requires passing "draw" as a subcommand to do the same
work. I ended up using the same hack in the new function I created, with
a reference to the original one, but I'm not sure this was the best
possible approach. Is there a cleaner way to solve this?

- Federico