[otlayout] Add HB_OT_LAYOUT_GLYPH_PROPS_LIGATED

Currently unused.
diff --git a/src/hb-ot-layout-private.hh b/src/hb-ot-layout-private.hh
index 1da4400..c37117b 100644
--- a/src/hb-ot-layout-private.hh
+++ b/src/hb-ot-layout-private.hh
@@ -50,8 +50,11 @@
   HB_OT_LAYOUT_GLYPH_PROPS_MARK		= 0x08u,
 
   /* The following are used internally; not derived from GDEF. */
-  HB_OT_LAYOUT_GLYPH_PROPS_SUBSTITUTED	= 0x10u
+  HB_OT_LAYOUT_GLYPH_PROPS_SUBSTITUTED	= 0x10u,
+  HB_OT_LAYOUT_GLYPH_PROPS_LIGATED	= 0x20u,
 
+  HB_OT_LAYOUT_GLYPH_PROPS_PRESERVE     = HB_OT_LAYOUT_GLYPH_PROPS_SUBSTITUTED |
+					  HB_OT_LAYOUT_GLYPH_PROPS_LIGATED
 } hb_ot_layout_glyph_class_mask_t;