[subset] Add new subsetting API method to receive a user specified glyph mapping.

src/hb-subset-input.cc

@@ -520,6 +520,22 @@ hb_subset_preprocess (hb_face_t *source)
   return new_source;
 }
 
+HB_EXTERN hb_bool_t
+hb_subset_input_set_old_to_new_glyph_mapping (hb_subset_input_t *input,
+                                              const hb_map_t* mapping)
+{
+  hb_set_t new_gids;
+  for (auto gid : mapping->values())
+  {
+    // Mapping cannot map multiple old gids to the same new gid.
+    if (new_gids.has(gid)) return false;
+    new_gids.add(gid);
+  }
+
+  input->glyph_map = *mapping;
+  return true;
+}
+
 #ifdef HB_EXPERIMENTAL_API
 /**
  * hb_subset_input_override_name_table:

src/hb-subset-input.hh

@@ -119,6 +119,7 @@ struct hb_subset_input_t
   bool force_long_loca = false;
 
   hb_hashmap_t<hb_tag_t, float> axes_location;
+  hb_map_t glyph_map;
 #ifdef HB_EXPERIMENTAL_API
   hb_hashmap_t<hb_ot_name_record_ids_t, hb_bytes_t> name_table_overrides;
 #endif

src/hb-subset.h

@@ -172,6 +172,11 @@ hb_subset_input_pin_axis_location (hb_subset_input_t  *input,
 				   hb_tag_t            axis_tag,
 				   float               axis_value);
 
+HB_EXTERN hb_bool_t
+hb_subset_input_set_old_to_new_glyph_mapping (hb_subset_input_t *input,
+                                              const hb_map_t* mapping);
+
+
 #ifdef HB_EXPERIMENTAL_API
 HB_EXTERN hb_bool_t
 hb_subset_input_override_name_table (hb_subset_input_t  *input,