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).