Refactor _hb_glyph_info_is_default_ignorable()

src/hb-ot-layout-private.hh

diff --git a/src/hb-ot-layout-private.hh b/src/hb-ot-layout-private.hh
index 47fecd2..d854563 100644
--- a/src/hb-ot-layout-private.hh
+++ b/src/hb-ot-layout-private.hh
@@ -225,10 +225,12 @@
   return info->unicode_props1();
 }
 
+static inline bool _hb_glyph_info_ligated (const hb_glyph_info_t *info);
+
 static inline hb_bool_t
 _hb_glyph_info_is_default_ignorable (const hb_glyph_info_t *info)
 {
-  return !!(info->unicode_props0() & MASK0_IGNORABLE);
+  return (info->unicode_props0() & MASK0_IGNORABLE) && !_hb_glyph_info_ligated (info);
 }
 
 static inline hb_bool_t