(window_scroll_pixel_based, Frecenter): Use

(window_scroll_pixel_based, Frecenter): Use
move_it_vertically_backward directly.
(Frecenter): Fix calculation of new start pos for negative arg.
Before, the new start pos was sometimes chosen too far back, so
the last line became only partially visible, and thus would be
either only semi-visible or automatically scrolled to the middle
of the window by redisplay.