(pos_visible_p): Remove exact_mode_line_heights_p arg;

(pos_visible_p): Remove exact_mode_line_heights_p arg;
so always calculate heights even when pos-visible-in-window-p is
called with partially = t.  Don't overshoot last_visible_y in move_it_to.
Return row height and row number in new rowh and vpos args.
(cursor_row_fully_visible_p): First line is always "fully visible".
(try_window): Don't clear matrix if vscrolled.