message: add screenshot example usage to astroblasto
diff: @@ -576,7 +576,10 @@ impl EventHandler for MainState {
  }
  Keycode::Space => {
  self.input.fire = true;
  - }
  + },
  + Keycode::P => {
  + graphics::screenshot(ctx, "screenshot.png");
  + },
  Keycode::Escape => ctx.quit().unwrap(),
  _ => (), // Do nothing
  }
  