patch 8.1.2176: syntax attributes not combined with Visual highlighting

Problem:    Syntax attributes not combined with Visual highlighting. (Arseny
            Nasokin)
Solution:   Combine the attributes. (closes #5083)
diff --git a/src/drawline.c b/src/drawline.c
index 333eeba..ef3b69f 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -1470,9 +1470,21 @@
 	    attr_pri = TRUE;
 #ifdef LINE_ATTR
 	    if (area_attr != 0)
+	    {
 		char_attr = hl_combine_attr(line_attr, area_attr);
+# ifdef FEAT_SYN_HL
+		if (syntax_attr != 0)
+		    char_attr = hl_combine_attr(syntax_attr, char_attr);
+# endif
+	    }
 	    else if (search_attr != 0)
+	    {
 		char_attr = hl_combine_attr(line_attr, search_attr);
+# ifdef FEAT_SYN_HL
+		if (syntax_attr != 0)
+		    char_attr = hl_combine_attr(syntax_attr, char_attr);
+# endif
+	    }
 # ifdef FEAT_TEXT_PROP
 	    else if (text_prop_type != NULL)
 	    {