patch 9.0.0445: when opening/closing window text moves up/down

Problem:    When opening/closing window text moves up/down.
Solution:   Add the 'splitscroll' option.  When off text will keep its
            position as much as possible.
diff --git a/src/structs.h b/src/structs.h
index c12f8e4..a477dc7 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3570,6 +3570,8 @@
     int		w_winrow;	    // first row of window in screen
     int		w_height;	    // number of rows in window, excluding
 				    // status/command/winbar line(s)
+    int		w_prev_winrow;	    // previous winrow used for 'splitscroll'
+    int		w_prev_height;	    // previous height used for 'splitscroll'
 
     int		w_status_height;    // number of status lines (0 or 1)
     int		w_wincol;	    // Leftmost column of window in screen.