Fix frozen remote gameOver ball

This adds a `gameOver` property to each `player` object on both the client and server. The remote player's ball is not rendered if that remote player has their `gameOver` property set to `true`. This prevents a frozen ball from being rendered at the bottom of the world bounds when a new player joins a game that has an existing player in the game over state.

This addresses Issue https://github.com/nsue/multiplayer-breakout/issues/18