[OT] Allow adding features with fallback implementation

src/hb-ot-map-private.hh

@@ -147,10 +147,10 @@ struct hb_ot_map_builder_t
 
   hb_ot_map_builder_t (void) { memset (this, 0, sizeof (*this)); }
 
-  HB_INTERNAL void add_feature (hb_tag_t tag, unsigned int value, bool global);
+  HB_INTERNAL void add_feature (hb_tag_t tag, unsigned int value, bool global, bool has_fallback = false);
 
-  inline void add_bool_feature (hb_tag_t tag, bool global = true)
-  { add_feature (tag, 1, global); }
+  inline void add_bool_feature (hb_tag_t tag, bool global = true, bool has_fallback = false)
+  { add_feature (tag, 1, global, has_fallback); }
 
   inline void add_gsub_pause (hb_ot_map_t::pause_func_t pause_func)
   { add_pause (0, pause_func); }
@@ -174,6 +174,7 @@ struct hb_ot_map_builder_t
     unsigned int seq; /* sequence#, used for stable sorting only */
     unsigned int max_value;
     bool global; /* whether the feature applies value to every glyph in the buffer */
+    bool has_fallback; /* whether to allocate bits even if feature not found */
     unsigned int default_value; /* for non-global features, what should the unset glyphs take */
     unsigned int stage[2]; /* GSUB/GPOS */
 

src/hb-ot-map.cc

@@ -60,7 +60,7 @@ hb_ot_map_t::add_lookups (hb_face_t    *face,
 }
 
 
-void hb_ot_map_builder_t::add_feature (hb_tag_t tag, unsigned int value, bool global)
+void hb_ot_map_builder_t::add_feature (hb_tag_t tag, unsigned int value, bool global, bool has_fallback)
 {
   feature_info_t *info = feature_infos.push();
   if (unlikely (!info)) return;
@@ -68,6 +68,7 @@ void hb_ot_map_builder_t::add_feature (hb_tag_t tag, unsigned int value, bool gl
   info->seq = feature_infos.len;
   info->max_value = value;
   info->global = global;
+  info->has_fallback = has_fallback;
   info->default_value = global ? value : 0;
   info->stage[0] = current_stage[0];
   info->stage[1] = current_stage[1];
@@ -175,6 +176,7 @@ hb_ot_map_builder_t::compile (hb_face_t *face,
 	  feature_infos[j].global = false;
 	  feature_infos[j].max_value = MAX (feature_infos[j].max_value, feature_infos[i].max_value);
 	}
+	feature_infos[j].has_fallback = feature_infos[j].has_fallback || feature_infos[i].has_fallback;
 	feature_infos[j].stage[0] = MIN (feature_infos[j].stage[0], feature_infos[i].stage[0]);
 	feature_infos[j].stage[1] = MIN (feature_infos[j].stage[1], feature_infos[i].stage[1]);
 	/* Inherit default_value from j */
@@ -209,7 +211,7 @@ hb_ot_map_builder_t::compile (hb_face_t *face,
 						   language_index[table_index],
 						   info->tag,
 						   &feature_index[table_index]);
-    if (!found)
+    if (!found && !info->has_fallback)
       continue;