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
fix pic.lua test spec

Since working on pic.lua, tests were broken.
char* to const char* in leptonica file writing interface

according to leptonica's headers, it is const char*. Just a cosmetic
change.
fix koptcontext test spec

API of PNG export functions has changed, reflect that in the test
suite.

Gets rid of an error message.
Merge pull request #289 from hwhw/master

refresh emulation, libk2pdfopt without mupdf linking