patch 9.0.1610: display is wrong when 'smoothscroll' is set

Problem:    Display is wrong when 'smoothscroll' is set and scrolling multiple
            lines.
Solution:   Redraw with UPD_NOT_VALID when "skipcol" is or was set.
            (closes #12490, closes #12468)
diff --git a/src/move.c b/src/move.c
index e021d71..4cd4d1c 100644
--- a/src/move.c
+++ b/src/move.c
@@ -1786,7 +1786,6 @@
 	int	    width1 = curwin->w_width - curwin_col_off();
 	int	    width2 = width1 + curwin_col_off2();
 	int	    size = 0;
-	linenr_T    prev_topline = curwin->w_topline;
 	colnr_T	    prev_skipcol = curwin->w_skipcol;
 
 	if (do_sms)
@@ -1850,10 +1849,9 @@
 	    }
 	}
 
-	// TODO: is comparing w_topline with prev_topline still needed?
-	if (curwin->w_topline == prev_topline
-		|| curwin->w_skipcol != prev_skipcol)
-	    // need to redraw because wl_size of the topline may now be invalid
+	if (prev_skipcol > 0 || curwin->w_skipcol > 0)
+	    // need to redraw more, because wl_size of the (new) topline may
+	    // now be invalid
 	    redraw_later(UPD_NOT_VALID);
     }
     else