patch 8.1.0105: all tab stops are the same

Problem:    All tab stops are the same.
Solution:   Add the variable tabstop feature. (Christian Brabandt,
            closes #2711)
diff --git a/src/screen.c b/src/screen.c
index b7b3e67..b5b68e4 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4848,8 +4848,14 @@
 			vcol_adjusted = vcol - MB_CHARLEN(p_sbr);
 #endif
 		    /* tab amount depends on current column */
+#ifdef FEAT_VARTABS
+		    tab_len = tabstop_padding(vcol_adjusted,
+					      wp->w_buffer->b_p_ts,
+					      wp->w_buffer->b_p_vts_array) - 1;
+#else
 		    tab_len = (int)wp->w_buffer->b_p_ts
-					- vcol_adjusted % (int)wp->w_buffer->b_p_ts - 1;
+			       - vcol_adjusted % (int)wp->w_buffer->b_p_ts - 1;
+#endif
 
 #ifdef FEAT_LINEBREAK
 		    if (!wp->w_p_lbr || !wp->w_p_list)