This optimizes the normal scrolling case; updates are done at the
bottom of the screen and then scrolled up.
In this case, the damage list will be a more or less sorted list of
updates, oldest first.
If we're scrolling a lot, the oldest updates will eventually scroll
off screen. In this case, there's no need to keep them in the damage
list.
So, when scrolling, loop the damage list and adjust/remove updates
that have scrolled off screen (either partially, or completely).
Stop looping as soon as we see an update that has *not* scrolled off
screen.
Note that this means there may be update items in the damage list
that *has* scrolled off screen. This is especially true for random
screen writes (i.e. typical to some synthetic benchmarks).
This adds a pointer to the first cell on the current line. This
pointer must be updated every time the row changes.
The advantage is mainly that PRINT doesn't have to call
grid_get_range(), which is fairly expensive.
Since horizontal cursor movement is clamped to the current line, we
can calculate the new linear cursor without any expensive
multiplications and/or divisions.
