Add reverse at waypoint orders.

The train will reverse when its tail is within the waypoint tile.
This is useful for reversing on train en-route, without creating
dedicated reversing sidings or platforms.

src/saveload/afterload.cpp

@@ -2951,6 +2951,13 @@ bool AfterLoadGame()
 		}
 	}
 
+	if (SlXvIsFeatureMissing(XSLFI_REVERSE_AT_WAYPOINT)) {
+		Train *t;
+		FOR_ALL_TRAINS(t) {
+			t->reverse_distance = 0;
+		}
+	}
+
 	/*
 	 * Only keep order-backups for network clients (and when replaying).
 	 * If we are a network server or not networking, then we just loaded a previously

src/saveload/extended_ver_sl.cpp

@@ -45,6 +45,7 @@ std::vector<uint32> _sl_xv_discardable_chunk_ids;           ///< list of chunks
 static const uint32 _sl_xv_slxi_chunk_version = 0;          ///< current version os SLXI chunk
 
 const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
+	{ XSLFI_REVERSE_AT_WAYPOINT,    XSCF_NULL,              1,                                    1, "reverse_at_waypoint",   NULL, NULL, NULL        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, NULL, NULL, NULL, NULL },// This is the end marker
 };
 

src/saveload/extended_ver_sl.h

@@ -21,6 +21,7 @@
  */
 enum SlXvFeatureIndex {
 	XSLFI_NULL                          = 0,      ///< Unused value, to indicate that no extended feature test is in use
+	XSLFI_REVERSE_AT_WAYPOINT,                    ///< Reverse at waypoint orders
 
 	XSLFI_SIZE,                                   ///< Total count of features, including null feature
 };

src/saveload/vehicle_sl.cpp

@@ -724,6 +724,7 @@ const SaveLoad *GetVehicleDescription(VehicleType vt)
 		SLE_CONDNULL(2, 2, 19),
 		 SLE_CONDVAR(Train, gv_flags,            SLE_UINT16,                 139, SL_MAX_VERSION),
 		SLE_CONDNULL(11, 2, 143), // old reserved space
+		SLE_CONDVAR_X(Train, reverse_distance,    SLE_UINT16,                   0, SL_MAX_VERSION, SlXvFeatureTest(XSLFTO_AND, XSLFI_REVERSE_AT_WAYPOINT)),
 
 		     SLE_END()
 	};