[API] Add hb_face_get_blob()

Need to think more about it.

src/hb-font.cc

@@ -685,6 +685,16 @@ hb_face_is_immutable (hb_face_t *face)
 }
 
 
+hb_blob_t *
+hb_face_get_blob (hb_face_t *face)
+{
+  if (face->destroy != (hb_destroy_func_t) _hb_face_for_data_closure_destroy)
+    return hb_blob_get_empty ();
+
+  hb_face_for_data_closure_t *data = (hb_face_for_data_closure_t *) face->user_data;
+  return data->blob;
+}
+
 hb_blob_t *
 hb_face_reference_table (hb_face_t *face,
 			 hb_tag_t   tag)

src/hb-font.h

@@ -79,6 +79,9 @@ hb_bool_t
 hb_face_is_immutable (hb_face_t *face);
 
 
+hb_blob_t *
+hb_face_get_blob (hb_face_t *face);
+
 hb_blob_t *
 hb_face_reference_table (hb_face_t *face,
 			 hb_tag_t   tag);