a "fast scroll" by setting the display start in memory.
(Only implemented for "scroll up" because this is more used. "scroll down"
is easy to add.)
This moves the semantics of "copyrows" to something like "moverows";
the contents of the new visible lines at the bottom is undefined.
The emulations can live without the original "copy" semantics.
The graphics device driver passes a "default attribute" for normal text
output to the wscons framework. If the emulation module needs more
attributes (for different "renditions") it can allocate them via a
callback.
For now, only the "sun" emulation makes use of it.
