Callbacks::draw() must not mutate the state

examples/example00.rs

@@ -9,7 +9,7 @@ impl Callbacks for Example {
     }
     fn handle_event(&mut self, ev: Event) {}
     fn update(&mut self) {}
-    fn draw(&mut self, graphics: &mut Graphics) {}
+    fn draw(&self, graphics: &mut Graphics) {}
     fn clean_up(self) {}
 }
 

src/lib.rs

@@ -21,7 +21,7 @@ pub trait Callbacks {
     }
     fn handle_event(&mut self, ev: Event);
     fn update(&mut self);
-    fn draw(&mut self, graphics: &mut Graphics);
+    fn draw(&self, graphics: &mut Graphics);
     fn clean_up(self);
 }