Add the new functions current_is_above_screen() (which doesn't account
for softwrapped chunks yet, but will when we can scroll edittop partially
off the screen, which is forthcoming), current_is_below_screen() (which
determines whether current[current_x] is past the softwrapped chunk at
the bottom of the screen), and current_is_offscreen() (the union of the
previous two functions).

edit_redraw() and edit_refresh() now use current_is_offscreen() to check
whether they should adjust the viewport, and adjust_viewport() now uses
current_is_above_screen() to determine whether current is on or below
the screen in FLOWING mode.