patch 8.1.1459: popup window border looks bad when 'ambiwidth' is "double"

Problem:    Popup window border looks bad when 'ambiwidth' is "double".
            (Yasuhiro Matsumoto)
Solution:   Only use line drawing characters when 'ambiwidth' is "single".
            (Ken Takata, closes #4477)

src/screen.c

@@ -1078,7 +1078,9 @@ update_popups(void)
 		+ wp->w_height + wp->w_popup_padding[2] + wp->w_popup_border[2];
 	popup_attr = get_wcr_attr(wp);
 
-	if (enc_utf8)
+	// We can only use these line drawing characters when 'encoding' is
+	// "utf-8" and 'ambiwidth' is "single".
+	if (enc_utf8 && p_ambw == 's')
 	{
 	    border_char[0] = border_char[2] = 0x2550;
 	    border_char[1] = border_char[3] = 0x2551;

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1459,
 /**/
     1458,
 /**/