fix night mode with viewport

was buggy after refactor
bugfix: logic on parameter test for viewport correction

would affect any device, though would never had triggered, probably.
For correctness sake, this fix.
emulated screen flash on Emulator

This (re-)introduces a simulated "flash" (for now: invert, invert back)
of refresh sections with the Emulator. Since this is only useful when
debugging, it is off by default. It can be enabled by setting an
environment variable EMULATE_READER_FLASH. Set it to the number of
microseconds you want the flash to take. Suggested are values 200-700,
depending on how closely you want to look at refreshes :-)
Merge pull request #282 from hwhw/master

Bug fixes, emulated screen flash