Camera doesn't move too low now

The losing player never crosses the upper rim.
Players warp above all bricks

So they don't warp into bricks anymore.
Players are thinner now

The sprites are only 16 pixels wide now. Collision is set to the whole
sprite so we can use `sprite_width` to estimate collision box.
An arrow shows an off-screen player

When a player is too high (above view), an arrow appears and shows her
horizontal position.
Also, turned off AI control.