Custom bridge heads: Add savegame version and setting.

src/saveload/afterload.cpp

@@ -2968,6 +2968,14 @@ bool AfterLoadGame()
 #endif
 	}
 
+	if (SlXvIsFeatureMissing(XSLFI_CUSTOM_BRIDGE_HEADS)) {
+		/* ensure that previously unused custom bridge-head bits are cleared */
+		for (TileIndex t = 0; t < map_size; t++) {
+			if (IsBridgeTile(t) && GetTunnelBridgeTransportType(t) == TRANSPORT_ROAD) {
+				SB(_m[t].m2, 0, 8, 0);
+			}
+		}
+	}
 
 	/* Station acceptance is some kind of cache */
 	if (IsSavegameVersionBefore(127)) {

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_CUSTOM_BRIDGE_HEADS,    XSCF_NULL,                1,   1, "custom_bridge_heads",       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_CUSTOM_BRIDGE_HEADS,                    ///< Custom bridge heads
 
 	XSLFI_SIZE,                                   ///< Total count of features, including null feature
 };