Add back API removed recently

Add hb_ot_layout_language_get_required_feature_index() again, which
is used in Pango.  This was removed in
da132937 in favor of
hb_ot_layout_language_get_required_feature().

API changes:

  - Added hb_ot_layout_language_get_required_feature_index back.

src/hb-ot-layout.cc

@@ -320,6 +320,21 @@ hb_ot_layout_script_find_language (hb_face_t    *face,
   return false;
 }
 
+hb_bool_t
+hb_ot_layout_language_get_required_feature_index (hb_face_t    *face,
+						  hb_tag_t      table_tag,
+						  unsigned int  script_index,
+						  unsigned int  language_index,
+						  unsigned int *feature_index)
+{
+  return hb_ot_layout_language_get_required_feature (face,
+						     table_tag,
+						     script_index,
+						     language_index,
+						     feature_index,
+						     NULL);
+}
+
 hb_bool_t
 hb_ot_layout_language_get_required_feature (hb_face_t    *face,
 					    hb_tag_t      table_tag,

src/hb-ot-layout.h

@@ -139,6 +139,13 @@ hb_ot_layout_script_find_language (hb_face_t    *face,
 				   hb_tag_t      language_tag,
 				   unsigned int *language_index);
 
+hb_bool_t
+hb_ot_layout_language_get_required_feature_index (hb_face_t    *face,
+						  hb_tag_t      table_tag,
+						  unsigned int  script_index,
+						  unsigned int  language_index,
+						  unsigned int *feature_index);
+
 hb_bool_t
 hb_ot_layout_language_get_required_feature (hb_face_t    *face,
 					    hb_tag_t      table_tag,