Zstd: Use zstd for network joins if supported at both ends

See also: https://github.com/OpenTTD/OpenTTD/pull/8773

src/network/network_client.cpp

@@ -482,6 +482,11 @@ NetworkRecvStatus ClientNetworkGameSocketHandler::SendGetMap()
 	my_client->status = STATUS_MAP_WAIT;
 
 	Packet *p = new Packet(PACKET_CLIENT_GETMAP);
+#if defined(WITH_ZSTD)
+	p->Send_bool(true);
+#else
+	p->Send_bool(false);
+#endif
 	my_client->SendPacket(p);
 	return NETWORK_RECV_STATUS_OKAY;
 }

src/network/network_server.cpp

@@ -654,7 +654,9 @@ NetworkRecvStatus ServerNetworkGameSocketHandler::SendMap()
 		sent_packets = 4; // We start with trying 4 packets
 
 		/* Make a dump of the current game */
-		if (SaveWithFilter(this->savegame, true, true) != SL_OK) usererror("network savedump failed");
+		SaveModeFlags flags = SMF_NET_SERVER;
+		if (this->supports_zstd) flags |= SMF_ZSTD_OK;
+		if (SaveWithFilter(this->savegame, true, flags) != SL_OK) usererror("network savedump failed");
 	}
 
 	if (this->status == STATUS_MAP) {
@@ -1098,6 +1100,8 @@ NetworkRecvStatus ServerNetworkGameSocketHandler::Receive_CLIENT_GETMAP(Packet *
 		return this->SendError(NETWORK_ERROR_NOT_AUTHORIZED);
 	}
 
+	this->supports_zstd = p->Recv_bool();
+
 	/* Check if someone else is receiving the map */
 	for (NetworkClientSocket *new_cs : NetworkClientSocket::Iterate()) {
 		if (new_cs->status == STATUS_MAP) {

src/network/network_server.h

@@ -75,6 +75,7 @@ public:
 	uint32 rcon_hash_bits;       ///< Rcon password hash entropy bits
 	uint32 settings_hash_bits;   ///< Settings password hash entropy bits
 	bool settings_authed = false;///< Authorised to control all game settings
+	bool supports_zstd = false;  ///< Client supports zstd compression
 
 	struct PacketWriter *savegame; ///< Writer used to write the savegame.
 	NetworkAddress client_address; ///< IP-address of the client (so he can be banned)