Add is_mutable() functions

Correspond to the make_mutable() ones.

src/hb-unicode.c

diff --git a/src/hb-unicode.c b/src/hb-unicode.c
index 19a891b..280c61f 100644
--- a/src/hb-unicode.c
+++ b/src/hb-unicode.c
@@ -108,6 +108,12 @@
   ufuncs->immutable = TRUE;
 }
 
+hb_bool_t
+hb_unicode_funcs_is_immutable (hb_unicode_funcs_t *ufuncs)
+{
+  return ufuncs->immutable;
+}
+
 
 void
 hb_unicode_funcs_set_mirroring_func (hb_unicode_funcs_t *ufuncs,