- In conjunction with all the below changes, attempt to fix A_CheckSightOrRange and A_CheckSight

for multiplayer: They now always check through the eyes of every player. For players whose
  cameras are not players, they also check through the eyes of those cameras.
- Using spynext/spyprev to switch from a non-player to a player now writes a command to the
  network stream and lets Net_DoCommand() take care of it later. The logic here is that if
  a player is viewing from something that isn't another player, then every player needs to know
  about it for sync purposes. Consequently, when they stop viewing from a non-player and switch
  to a player, everybody needs to know about that too. But if they are viewing from a
  player, it doesn't matter which player it is, so they can spynext/spyprev all they want
  without letting the other players know about it (and without potentially breaking demos--due
  to the above-mentioned two codepointers--while doing it during demo playback).
- Replaced the instances of checking players[consoleplayer].camera for a valid pointer to
  ones that do it for every player.
- Fixed: Upon changing levels, all players but the consoleplayer would have their cameras NULLed.
- Fixed: player_t::FixPointers() needs to bypass the read barriers, or it won't be able to
  do substitutions of old objects that are pending deletion.

SVN r3448 (trunk)

src/g_game.cpp

@@ -831,7 +831,12 @@ static void ChangeSpy (bool forward)
 	// If not viewing through a player, return your eyes to your own head.
 	if (players[consoleplayer].camera->player == NULL)
 	{
-		players[consoleplayer].camera = players[consoleplayer].mo;
+		// When watching demos, you will just have to wait until your player
+		// has done this for you, since it could desync otherwise.
+		if (!demoplayback)
+		{
+			Net_WriteByte(DEM_REVERTCAMERA);
+		}
 		return;
 	}