refresh emulation: only update areas that are refreshed

when refresh emulation is enabled, only the parts of the screen
are actually updated that are being refreshed. This allows to
debug refresh regions with the emulator.
Reminder: activate refresh emulation by setting the environment
variable EMULATE_READER_FLASH to the number of milliseconds that
you want the inverse flash to endure. Suggested are now about
100 (msecs).
talk about new emulator options in the README
no debug output from MuPDF FFI wrapper by default

before, when no debug function was set up, the wrapper used print()
do show debug info on console. It is now up to the caller to set a
proper debug function for MuPDF if debug output is whished.
Merge pull request #287 from hwhw/master

refresh emulation: only update areas that are refreshed