patch 9.0.1603: display wrong if scrolling multiple lines with 'smoothscroll'

Problem:    Display wrong when scrolling multiple lines with 'smoothscroll'
            set.
Solution:   Redraw when w_skipcol changed. (closes #12477, closes #12468)
diff --git a/src/move.c b/src/move.c
index ff2f494..54f5648 100644
--- a/src/move.c
+++ b/src/move.c
@@ -1785,6 +1785,7 @@
 	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)
 	    size = linetabsize(curwin, curwin->w_topline);
@@ -1847,8 +1848,9 @@
 	    }
 	}
 
-	if (curwin->w_topline == prev_topline)
-	    // need to redraw even though w_topline didn't change
+	if (curwin->w_topline == prev_topline
+		|| curwin->w_skipcol != prev_skipcol)
+	    // need to redraw because wl_size of the topline may now be invalid
 	    redraw_later(UPD_NOT_VALID);
     }
     else