Make atomic types' internal values non-mutable

This resulted from confusion previously...

src/hb-atomic.hh

@@ -267,14 +267,14 @@ inline void *hb_atomic_ptr_impl_get (void **P)	{ void *v = *P; _hb_memory_r_barr
 #define HB_ATOMIC_INT_INIT(V)          {V}
 struct hb_atomic_int_t
 {
-  inline void set_relaxed (int v_) const { hb_atomic_int_impl_set_relaxed (&v, v_); }
-  inline void set (int v_) const { hb_atomic_int_impl_set (&v, v_); }
+  inline void set_relaxed (int v_) { hb_atomic_int_impl_set_relaxed (&v, v_); }
+  inline void set (int v_) { hb_atomic_int_impl_set (&v, v_); }
   inline int get_relaxed (void) const { return hb_atomic_int_impl_get_relaxed (&v); }
   inline int get (void) const { return hb_atomic_int_impl_get (&v); }
   inline int inc (void) { return hb_atomic_int_impl_add (&v,  1); }
   inline int dec (void) { return hb_atomic_int_impl_add (&v, -1); }
 
-  mutable int v;
+  int v;
 };
 
 
@@ -285,7 +285,7 @@ struct hb_atomic_ptr_t
   typedef typename hb_remove_pointer<P>::value T;
 
   inline void init (T* v_ = nullptr) { set_relaxed (v_); }
-  inline void set_relaxed (T* v_) const { hb_atomic_ptr_impl_set_relaxed (&v, v_); }
+  inline void set_relaxed (T* v_) { hb_atomic_ptr_impl_set_relaxed (&v, v_); }
   inline T *get_relaxed (void) const { return (T *) hb_atomic_ptr_impl_get_relaxed (&v); }
   inline T *get (void) const { return (T *) hb_atomic_ptr_impl_get ((void **) &v); }
   inline bool cmpexch (const T *old, T *new_) const { return hb_atomic_ptr_impl_cmpexch ((void **) &v, (void *) old, (void *) new_); }
@@ -293,7 +293,7 @@ struct hb_atomic_ptr_t
   inline T * operator -> (void) const { return get (); }
   template <typename C> inline operator C * (void) const { return get (); }
 
-  mutable T *v;
+  T *v;
 };
 
 

src/hb-object.hh

@@ -194,7 +194,7 @@ struct hb_user_data_array_t
 struct hb_object_header_t
 {
   hb_reference_count_t ref_count;
-  hb_atomic_int_t writable;
+  mutable hb_atomic_int_t writable;
   hb_atomic_ptr_t<hb_user_data_array_t> user_data;
 };
 #define HB_OBJECT_HEADER_STATIC \