Expand `player_state`, add a `passive` state

[Tianjian]

Currently, player_state has the following members:

enum class player_state
{
  active,
  dead
};

My proposed change:

enum class player_state
{
  active, // a normal state
  passive, // a state that the player is non-interactable
  dead // deactivated
};

A passive state is that a player is invulnerable and cannot interact with the other players/enviroment, we should distinguish this from the real invulnerable state. When a player shrinks, it should become passive, not only invulnerable.