Add is_mutable() functions

Correspond to the make_mutable() ones.

src/hb-font.cc

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,

src/hb-font.h

diff --git a/src/hb-font.h b/src/hb-font.h
index a7b6d8f..266b74a 100644
--- a/src/hb-font.h
+++ b/src/hb-font.h
@@ -91,6 +91,8 @@
 void
 hb_font_funcs_make_immutable (hb_font_funcs_t *ffuncs);
 
+hb_bool_t
+hb_font_funcs_is_immutable (hb_font_funcs_t *ffuncs);
 
 /* funcs */
 

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,

src/hb-unicode.h

diff --git a/src/hb-unicode.h b/src/hb-unicode.h
index a84a948..4edab48 100644
--- a/src/hb-unicode.h
+++ b/src/hb-unicode.h
@@ -201,6 +201,8 @@
 void
 hb_unicode_funcs_make_immutable (hb_unicode_funcs_t *ufuncs);
 
+hb_bool_t
+hb_unicode_funcs_is_immutable (hb_unicode_funcs_t *ufuncs);
 
 /*
  * funcs