Pretending that the manager starts in the overlay mode was really
confusing.
Also, the concept of _workScreen seemed great in theory but lead to
various subtle errors which were very hard to debug (related to cursor
handling, resolution changing etc).
- seamless resolution/palette switching (at least in theory, it still
isn't artifact-free, no clue why)
- bugfixes (shake offset, some s_screenSurf corner cases)
- simplification the triple-buffering code
