Add back API removed recently

Add hb_ot_layout_language_get_required_feature_index() again, which
is used in Pango.  This was removed in
da132937989acb4d8ca9bd41c79f98750e7dda30 in favor of
hb_ot_layout_language_get_required_feature().

API changes:

  - Added hb_ot_layout_language_get_required_feature_index back.