Add whole-map savegame chunk

This is to avoid haveing to do many passes to save/load map

src/saveload/extended_ver_sl.cpp

@@ -84,6 +84,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_TRAIN_FLAGS_EXTRA,      XSCF_NULL,                1,   1, "train_flags_extra",         NULL, NULL, NULL        },
 	{ XSLFI_TRAIN_THROUGH_LOAD,     XSCF_NULL,                1,   1, "train_through_load",        NULL, NULL, NULL        },
 	{ XSLFI_ORDER_EXTRA_DATA,       XSCF_NULL,                1,   1, "order_extra_data",          NULL, NULL, NULL        },
+	{ XSLFI_WHOLE_MAP_CHUNK,        XSCF_NULL,                1,   1, "whole_map_chunk",           NULL, NULL, "WMAP"      },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, NULL, NULL, NULL, NULL },// This is the end marker
 };