- 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/p_mobj.cpp

@@ -4051,9 +4051,12 @@ APlayerPawn *P_SpawnPlayer (FMapThing *mthing, bool tempplayer)
 
 	p->velx = p->vely = 0;		// killough 10/98: initialize bobbing to 0.
 
-	if (players[consoleplayer].camera == oldactor)
+	for (int ii = 0; ii < MAXPLAYERS; ++ii)
 	{
-		players[consoleplayer].camera = mobj;
+		if (playeringame[ii] && players[ii].camera == oldactor)
+		{
+			players[ii].camera = oldactor;
+		}
 	}
 
 	// [RH] Allow chasecam for demo watching