Add a MUPDF option under PDFTOPS_RENDERER. When activated, mupdf will be used
instead of Ghostscript/Poppler. When inactive, mupdf is not a dependency.

PR: 217848
Submitted by: raivo.hool@gmail.com
Reviewed by: mat
Approved by: swills (mentor), cyberbotx@cyberbotx.com (maintainer)
Differential Revision: https://reviews.freebsd.org/D10325