Merge pull request #287 from hwhw/master

refresh emulation: only update areas that are refreshed
even more refined refresh emulation - on SDL1.2 for now
do not use mupdf in libk2pdfopt

This will stop linking and (trying to) using mupdf in libk2pdfopt.
We didn't use the functionality anyway, and it produced funny error
messages (see #288, #286).

Needs https://github.com/koreader/libk2pdfopt/pull/24

Fixes #288