multiplayer stage 2: peer identity, stable vehicle ids and a session lobby

TTrain::id() numbers cabs with a local counter, in the order each peer happens
to create them, so it cannot address a vehicle across the session. Neither was
there any way for a player to see what is available to drive.

- network/entities: PeerId (the host is peer 1, remote peers are numbered from
  2, so it is a participant rather than a special case) and NetworkEntityId,
  plus a registry that numbers the drivable vehicles of the loaded scenario in
  name order and resolves ids back to TDynamicObject
- the server owns the numbering and publishes it as VEHICLE_LIST - name, crew
  occupancy, crew capacity, AI state, availability - on every change and at
  least every 120 frames, out of band so it stays out of the replay backbuffer;
  a client binds the received ids to its own copy of the scenario by name
- SERVER_HELLO hands the joining peer its PeerId
- a multiplayer lobby listing that roster, with vehicles taken by other players
  shown as such rather than hidden; entering one goes through the existing
  entervehicle command, the vehicle is never moved

The lobby lives inside the driver mode instead of a mode of its own: the world
has to keep running while somebody is choosing (a client stops consuming the
server's frames the moment its mode is not a command processor), and the same
screen is what a player needs to change vehicles later on.

Crew occupancy is still derived from the presence of a cab instance; the real
crew model, claim handshake and AI takeover follow in stage 3.

widgets/multiplayer_lobby.cpp

@@ -0,0 +1,119 @@
+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+#include "stdafx.h"
+#include "widgets/multiplayer_lobby.h"
+
+#include "application/application.h"
+#include "network/entities.h"
+#include "simulation/simulation.h"
+#include "utilities/Globals.h"
+#include "utilities/utilities.h"
+#include "utilities/translation.h"
+#include "vehicle/Driver.h"
+#include "vehicle/DynObj.h"
+#include "vehicle/Train.h"
+
+ui::multiplayer_lobby_panel::multiplayer_lobby_panel()
+    : ui_panel(STR_C("Multiplayer lobby"), false)
+{
+	size_min = {480, 260};
+}
+
+void ui::multiplayer_lobby_panel::enter_vehicle(std::string const &Name)
+{
+	TDynamicObject *vehicle = simulation::Vehicles.find(Name);
+	if (vehicle == nullptr)
+		return;
+
+	std::string payload{Name};
+	// the request travels as an ordinary simulation command, so every peer builds the cab
+	m_relay.post(user_command::entervehicle, 0.0, simulation::Train ? simulation::Train->id() : 0, GLFW_PRESS, 0, vehicle->GetPosition(), &payload);
+	// ...and the driver mode moves our own camera in once the cab shows up locally
+	Global.network_pending_vehicle = Name;
+	is_open = false;
+}
+
+void ui::multiplayer_lobby_panel::render_contents()
+{
+	bool const host{Application.is_server()};
+	bool const client{Application.is_client()};
+
+	if (!host && !client)
+	{
+		ImGui::TextUnformatted(STR_C("This is not a multiplayer session."));
+		return;
+	}
+
+	ImGui::Text("%s: %s, peer %u", STR_C("Session"), host ? (client ? "host + client" : "host") : "client", Global.network_peer_id);
+	ImGui::TextUnformatted(Global.SceneryFile.c_str());
+	ImGui::Separator();
+
+	auto const &entries = network::Entities.entries();
+	if (entries.empty())
+	{
+		ImGui::TextUnformatted(STR_C("Waiting for the vehicle list from the server..."));
+		return;
+	}
+
+	std::string const currentvehicle{simulation::Train != nullptr && simulation::Train->Dynamic() != nullptr ? simulation::Train->Dynamic()->name() : std::string()};
+
+	if (!FreeFlyModeFlag)
+	{
+		ImGui::TextUnformatted(STR_C("Leave the cab to change vehicles."));
+		ImGui::Separator();
+	}
+
+	ImGui::Columns(4, "mp_vehicles", false);
+	ImGui::SetColumnWidth(1, 80.0f * Global.ui_scale);
+	ImGui::SetColumnWidth(2, 90.0f * Global.ui_scale);
+	ImGui::SetColumnWidth(3, 130.0f * Global.ui_scale);
+
+	ImGui::TextDisabled("%s", STR_C("Vehicle"));
+	ImGui::NextColumn();
+	ImGui::TextDisabled("%s", STR_C("Crew"));
+	ImGui::NextColumn();
+	ImGui::TextDisabled("%s", STR_C("Driver"));
+	ImGui::NextColumn();
+	ImGui::NextColumn();
+	ImGui::Separator();
+
+	for (auto const &entry : entries)
+	{
+		ImGui::PushID(static_cast<int>(entry.id));
+
+		ImGui::TextUnformatted(entry.name.c_str());
+		ImGui::NextColumn();
+
+		ImGui::Text("%u/%u", (unsigned)entry.crew_count, (unsigned)entry.crew_capacity);
+		ImGui::NextColumn();
+
+		ImGui::TextUnformatted(entry.crew_count > 0 ? STR_C("player") : entry.ai_active ? STR_C("AI") : STR_C("free"));
+		ImGui::NextColumn();
+
+		if (entry.name == currentvehicle)
+		{
+			ImGui::TextUnformatted(STR_C("you"));
+		}
+		else if (entry.claimable && FreeFlyModeFlag && network::Entities.resolve(entry.id) != nullptr)
+		{
+			if (ImGui::Button(STR_C("Enter"), ImVec2(-1, 0)))
+				enter_vehicle(entry.name);
+		}
+		else
+		{
+			ImGui::TextDisabled("%s", entry.claimable ? "-" : STR_C("occupied"));
+		}
+		ImGui::NextColumn();
+
+		ImGui::PopID();
+	}
+
+	ImGui::Columns(1);
+}