crengine: "code" elements are not block elements, but inline elements
update crengine: fix <code> element

(needs https://github.com/koreader/crengine/pull/5)
Merge pull request #284 from hwhw/epub_code

crengine: "code" elements are not block elements, but inline elements
refresh queuing

This replaces the wait-for-refresh (or "update") logic with a
queue that tracks the updates and will wait only for those updates
that cover overlapping sections with an update newly scheduled.

Adds some debugging output, too.
Merge pull request #285 from hwhw/refresh_tracking

refresh queuing
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).