Setting a variable called game_mode to "over" is great, but it doesn't actually make anything else happen.

We need to act on the game_mode.

Thinking about it, this game is going to have three distinct modes:

• Pre-start - when it first loads, the game is going to display "welcome to flappy birds" or something, and "click to start". Otherwise it would just start without warning and the bird's probably going to hit the ground before the player knows what's going on.
• Playing - all the normal stuff: the bird goes up and down, the pipes slide past, and we check for collisions.
• Over - the pipes stop sliding and we display a "game over" message and maybe a score.

Depending on the mode, we'll make Do_a_Frame do slightly different things.

We'll also do slightly different things in the user-input handler.