Update print/mupdf to 1.8.

pkgsrc changes:
* Bump BUILDLINK_API_DEPENDS (spotted and tested via the only print/mupdf
  consumer wip/zathura-mupdf)
* OpenGL-based desktop viewer is still not enabled by default neither upstream
  nor in pkgsrc

Changes:
List of changes in MuPDF 1.8
* Headline changes:
* New OpenGL-based barebones desktop viewer.
* New URW fonts with greek and cyrillic.
* 64-bit file support.
* Ghostscript proofing mode (source only; not in shipped binaries).
* EPUB improvements:
* User style sheets.
* GIF images (also for CBZ).
* Table of contents.
* CJK text.
* Page margins.
* Many bug fixes.
* Bug fixes:
* Updated FreeType to version 2.6.1.
* Various font substitution bug fixes.
* Fix setjmp/longjmp behaviour which could cause optimizing compilers to misbehave.

(leot)