multiplayer: two sync margins in the ini, and the session decides the trainsets

Two hardcoded constants decided how far a train could drift before it was put
back, and there was no way to change them short of a rebuild. They are settings
now:

    multiplayer.sync.running  1.5   // margin while the train is rolling
    multiplayer.sync.stop     0.2   // one precise correction once it stands

The split is what the two numbers are for. A moving train is left to its own
physics unless it is well out, because nudging it several times a second is
worse than the error; a standing one is worth getting right to the centimetre,
and it is done once and then left alone until it moves again - a consist that
has had its correction is marked settled, and clears that mark the moment it
rolls. Both travel in SERVER_HELLO: whatever a player has in their own ini, a
session runs to the host's margins, which is the only way the peers agree on
what "out of place" means.

And the trainsets. Every peer loads its own copy of the scenery - that is the
only reason joining takes seconds instead of minutes - but the copy on disk is
not the session: the host may have shunted a set apart, put one together, or be
running a scenery whose consists were edited since. So the composition travels
in a full snapshot, ahead of the positions: which coupler of which vehicle is
joined to which coupler of which other, and with what hoses and cables. A
joining peer breaks what should not be coupled, makes what should, and only
then gets told where everything is - so the per-consist repositioning works on
the session's trains rather than on the ones its own scenery file described.

The vehicles themselves do not travel, and cannot: a peer missing one gets a
line in the log saying so, not a model over the wire.

Uncoupling has to be forced here. TDynamicObject::Dettach() leaves the
mechanical coupler alone unless the buffers are pressed together, which is
right for a player pulling a lever and wrong for putting a peer's world in
order.

Protocol version 5, snapshot version 6.

utilities/Globals.cpp

@@ -902,6 +902,18 @@ bool global_settings::ConfigParseNetwork(cParser& Parser, const std::string& tok
         return true;
     }
 
+    if (token == "multiplayer.sync.running")
+    {
+        ParseOneClamped(Parser, multiplayer_sync_running, 0.05f, 100.0f, 1, false);
+        return true;
+    }
+
+    if (token == "multiplayer.sync.stop")
+    {
+        ParseOneClamped(Parser, multiplayer_sync_stop, 0.01f, 100.0f, 1, false);
+        return true;
+    }
+
     return false;
 }
 
@@ -1815,6 +1827,8 @@ global_settings::export_as_text( std::ostream &Output ) const {
             << "network.client "
             << network_client->first << " " << network_client->second << "\n";
     }
+    export_as_text( Output, "multiplayer.sync.running", multiplayer_sync_running );
+    export_as_text( Output, "multiplayer.sync.stop", multiplayer_sync_stop );
     export_as_text( Output, "execonexit", exec_on_exit );
 }
 

utilities/Globals.h

@@ -372,6 +372,14 @@ struct global_settings {
 	uint64_t network_session_token = 0;
 	// how far the worst vehicle was out of place when the last correction arrived, metres
 	float network_position_error = 0.0f;
+	// how far a train may be from where the session says it is before it gets put back,
+	// in metres. the first applies while it is rolling, where nudging it too eagerly is
+	// worse than the error itself; the second once it has come to a stand, where it is
+	// worth getting exactly right and is done once rather than every update.
+	// the server hands its own values to every client on connect, so a session is
+	// consistent whatever each player has in their ini
+	float multiplayer_sync_running = 1.5f;
+	float multiplayer_sync_stop = 0.2f;
 	// the scenario has finished loading and the world can be worked with. this is not the
 	// same as simulation::is_ready, which only goes up once the player has a cab
 	bool simulation_loaded = false;