fix laggy event bug; expose window struct members

I think the window members need to be exposed so that we can do things like

    fn mouse_move(&mut self, w: &mut GameWindowSDL2, args: &MouseMoveArgs) {
      mouse::warp_mouse_in_window(
        &w.render_window.window,
        WINDOW_WIDTH as i32 / 2,
        WINDOW_HEIGHT as i32 / 2
      );
    }