Use go_back_chunks() and go_forward_chunks() to adjust edittop and to
move up or down to the scrolled region before updating the rows there.
Now softwrap mode and non-softwrap mode will behave the same way when
we can scroll the top line of the screen partially off the screen,
which is forthcoming.

(Note that the top line of the screen can't be partially scrolled yet,
so we have to work around that for now.)