Initialize mutexes in _cairo_ft_unscaled_font_map_lock()

This function is reachable via cairo_ft_font_face_create_for_ft_face()
-> _cairo_ft_unscaled_font_create_from_face() ->
_cairo_ft_unscaled_font_create_internal() ->
_cairo_ft_unscaled_font_map_lock(). Thus, it could happen that nothing
initialized mutexes before this code runs.

Signed-off-by: Uli Schlachter <psychon@znc.in>

src/cairo-ft-font.c

@@ -370,6 +370,8 @@ _cairo_ft_unscaled_font_map_destroy (void)
 static cairo_ft_unscaled_font_map_t *
 _cairo_ft_unscaled_font_map_lock (void)
 {
+    CAIRO_MUTEX_INITIALIZE ();
+
     CAIRO_MUTEX_LOCK (_cairo_ft_unscaled_font_map_mutex);
 
     if (unlikely (cairo_ft_unscaled_font_map == NULL)) {