Add hb_buffer_serialize_list_formats()

src/hb-buffer.cc

diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index a599153..4ac1fbe 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -1035,6 +1035,12 @@
   NULL
 };
 
+const char **
+hb_buffer_serialize_list_formats (void)
+{
+  return serialize_formats;
+}
+
 hb_buffer_serialize_format_t
 hb_buffer_serialize_format_from_string (const char *str, int len)
 {

src/hb-buffer.h

diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index ba9cd1d..db9b40f 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -259,6 +259,9 @@
 const char *
 hb_buffer_serialize_format_to_string (hb_buffer_serialize_format_t format);
 
+const char **
+hb_buffer_serialize_list_formats (void);
+
 /* Returns number of items, starting at start, that were serialized. */
 unsigned int
 hb_buffer_serialize_glyphs (hb_buffer_t *buffer,