This is largely untested, but existing scrolling code has been
converted to using a terminal-global scrolling region that is defined
as start-end of the scrollable region.
This is compared to the old code where the scrolling region where
defined in terms of marginals, counted in lines from top and from
bottom.
Instead of having each cell in the grid track it's own dirtiness, grid
operations now append "damage" to a list.
This list is consumed every time we render the grid.
This allows us to special case some operations, like erase (and in the
future, scroll).
