Added Scroller_TwinMode and Scroller_SingleScoreLives

Define Scroller_TwinMode to enable both players but they are controlled
by one stick.
Both players have independent collision.
Consider enabling Scroller_SingleScoreLives at the same time. It isn't
mandatory though. :)

Define Scroller_SingleScoreLives so there is one score and lives
displayed even if two players are active.
In other words the life count and score is shared. When both players
have died then the game ends.
Most often used with Scroller_LinkedPlayers or Scroller_TwinMode.