Merge pull request #290 from hwhw/master

Revert "do not use mupdf in libk2pdfopt"
get rid of the annoying 'cannot open' message when reflowing
Merge pull request #291 from koreader/chrox-master

get rid of the annoying 'cannot open' message when reflowing
avoid doing zero-size refreshes

Those refresh requests will crash (at least) Kobos' framebuffer
driver and create the image of a locked-up device.
Beware: device is not actually locked up and does in fact react
to touch screen events!