Add callback function to allow resize messages to be sent on mac.

src/x11/headless.rs

@@ -52,6 +52,9 @@ impl HeadlessContext {
     pub fn get_api(&self) -> ::Api {
         ::Api::OpenGl
     }
+
+    pub fn set_window_resize_callback(&mut self, _: fn(uint, uint)) {
+    }
 }
 
 impl Drop for HeadlessContext {