Add _hb_glyph_info_is_unicode_mark()

Unused right now.

src/hb-ot-layout-private.hh

diff --git a/src/hb-ot-layout-private.hh b/src/hb-ot-layout-private.hh
index 80a8854..8e6164e 100644
--- a/src/hb-ot-layout-private.hh
+++ b/src/hb-ot-layout-private.hh
@@ -267,6 +267,12 @@
   return (hb_unicode_general_category_t) (info->unicode_props() & UPROPS_MASK_GEN_CAT);
 }
 
+static inline bool
+_hb_glyph_info_is_unicode_mark (const hb_glyph_info_t *info)
+{
+  return HB_UNICODE_GENERAL_CATEGORY_IS_MARK (info->unicode_props() & UPROPS_MASK_GEN_CAT);
+}
+
 static inline void
 _hb_glyph_info_set_modified_combining_class (hb_glyph_info_t *info,
 					     unsigned int modified_class)