patch 8.2.1556: cursorline highlighting always overrules sign highlighting

Problem:    Cursorline highlighting always overrules sign highlighting.
Solution:   Combine the highlighting, use the priority to decide how.
            (closes #6812)
diff --git a/src/structs.h b/src/structs.h
index 38ebcf5..528c247 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -817,6 +817,7 @@
     char_u	*sat_text;
     int		sat_texthl;
     int		sat_linehl;
+    int		sat_priority;
 } sign_attrs_T;
 
 #if defined(FEAT_SIGNS) || defined(PROTO)