New package: openjpeg2-2.1.0

This is version 2.x of openjpeg – as the soname is libopenjpeg2, and
most other distributions package it as openjpeg2 as well, we'll call it
openjpeg2, instead of bumping the openjpeg package, which tracks the 1.x
release.
zathura-pdf-mupdf: actually build a working plugin