Fix PDF scanner + support image extraction

[cameron-dunn-sublime]

xref_length was erroring on the old version of PyMuPDF.

mupdf_display_errors errored with the new version, so it was removed.

Image extraction from PDFs is a new feature.

I don't know why GitHub says that I'm merging 6 commits. Local git log:

nothing added to commit but untracked files present (use "git add" to track)
➜  strelka git:(cd.images-from-pdf) ✗  glg
commit cda2495b24c2661f251288ea3ec1f191ade39bc9 (HEAD -> cd.images-from-pdf, sublime/cd.images-from-pdf)
Author: Cameron Dunn <[email protected]>
Date:   Wed Oct 13 18:57:47 2021 -0700

    Fix PDF scanner + support image extraction

    xref_length was erroring on the old version of PyMuPDF.

    mupdf_display_errors errored with the new version, so it was removed.

    Image extraction from PDFs is a new feature.

 build/python/backend/requirements.txt   |  2 +-
 src/python/strelka/scanners/scan_pdf.py | 19 ++++++++++++++++---
 2 files changed, 17 insertions(+), 4 deletions(-)

commit d9086f35d709592733ff690ed3a9ddeff5bbb433 (sublime/master, origin/master, origin/HEAD, master)
Author: Paul Hutelmyer <[email protected]>
Date:   Tue Oct 12 08:12:36 2021 -0400

It shouldn't matter though.

[cameron-dunn-sublime]

@jkamdjou this is the change I made while we were pairing the other day. I'll probably try and cleanup further and give a PR to target/strelka but in the mean time we can commit this to our [public] fork.