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/move.c b/src/move.c
index 9ec7798..8660d89 100644
--- a/src/move.c
+++ b/src/move.c
@@ -981,7 +981,8 @@
     /*
      * First make sure that w_topline is valid (after moving the cursor).
      */
-    update_topline();
+    if (p_spsc)
+	update_topline();
 
     /*
      * Next make sure that w_cline_row is valid.