[MP] Fix relay protocol routing with negative targets

[Faless]

Godot supports sending messages to "all but one peer" by sending a packet with a negative target (the negated ID of the excluded peer).

The relay protocol was incorrectly interpreting the values and relaying the message to the wrong peers.

This issue only affected "send_bytes" since the other subsystem (RPC and replication) "resolves" the correct IDs client-side (to match visibility information).

Tested with the following script:

extends Control

var last_id = 1

# Called when the node enters the scene tree for the first time.
func _ready() -> void:
	multiplayer.connected_to_server.connect(_connected)
	var peer = ENetMultiplayerPeer.new()
	if "--server" in OS.get_cmdline_args():
		peer.create_server(4343)
	else:
		peer.create_client("127.0.0.1", 4343)
	multiplayer.multiplayer_peer = peer
	multiplayer.peer_packet.connect(f1)
	multiplayer.peer_connected.connect(_peer_connected)


func _peer_connected(id):
	if id != 1:
		last_id = id


func _connected():
	await get_tree().create_timer(3).timeout
	(multiplayer as SceneMultiplayer).send_bytes(var_to_bytes(last_id), last_id)
	(multiplayer as SceneMultiplayer).send_bytes(var_to_bytes(-last_id), -last_id)
	(multiplayer as SceneMultiplayer).send_bytes(var_to_bytes(1), 1)
	(multiplayer as SceneMultiplayer).send_bytes(var_to_bytes(0), 0)
	(multiplayer as SceneMultiplayer).send_bytes(var_to_bytes(-1), -1)


func f1(from=0, data=PackedByteArray()):
	print("Called f1 from %d=%d in %d. Data: %d" % [multiplayer.get_remote_sender_id(), from, multiplayer.get_unique_id(), bytes_to_var(data)])

[akien-mga]

Thanks!