zathura-pdf-mupdf (zathura support for mupdf backend)

The zathura-pdf-mupdf plugin adds PDF support to zathura by using the
mupdf rendering library (replacing the poppler-based PDF support from
the zathura package).

Note: This overwrites /usr/lib/zathura/pdf.so from the zathura
package (replace lib with lib64 for 64-bit systems). zathura doesn't
support having multiple plugins per filetype, so after installing
zathura-pdf-mupdf, zathura will always use the mupdf backend. To switch
back to the poppler backend, remove both zathura and zathura-pdf-mupdf,
then reinstall zathura.

Note: Make sure your mupdf package has been upgraded to at least version
1.2 before building this.
