Don't reallocate the backbuffer on every refresh (thx sECuRE)

i3bar/include/xcb.h

@@ -56,6 +56,12 @@ void get_atoms();
  */
 void destroy_window(i3_output *output);
 
+/*
+ * Reallocate the statusline-buffer
+ *
+ */
+void realloc_sl_buffer();
+
 /*
  * Reconfigure all bars and create new for newly activated outputs
  *