Rework handling of requiredFeature to solve problem with rlig in arial.ttf from winxp https://bugzilla.mozilla.org/show_bug.cgi?id=986802 Fixes https://github.com/behdad/harfbuzz/pull/39 API Change: -hb_ot_layout_language_get_required_feature_index +hb_ot_layout_language_get_required_feature New API takes an extra pointer argument. Pass NULL in to get behavior of previous API. Reworked by behdad