Add is_mutable() functions

Correspond to the make_mutable() ones.
diff --git a/src/hb-font.cc b/src/hb-font.cc
index e257cd8..fd5df93 100644
--- a/src/hb-font.cc
+++ b/src/hb-font.cc
@@ -141,6 +141,12 @@
   ffuncs->immutable = TRUE;
 }
 
+hb_bool_t
+hb_font_funcs_is_immutable (hb_font_funcs_t *ffuncs)
+{
+  return ffuncs->immutable = TRUE;
+}
+
 
 void
 hb_font_funcs_set_glyph_func (hb_font_funcs_t *ffuncs,