Add an extended version feature for map sizes > 8kx8k.

This is to prevent earlier versions from trying to load the savegame
and aborting.

src/saveload/extended_ver_sl.cpp

@@ -32,6 +32,7 @@
 #include "../debug.h"
 #include "saveload.h"
 #include "extended_ver_sl.h"
+#include "../map_func.h"
 
 #include <vector>
 
@@ -45,6 +46,7 @@ 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_EXTRA_LARGE_MAP,        XSCF_NULL,              0,                                    1, "extra_large_map",       NULL, NULL, NULL        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, NULL, NULL, NULL, NULL },// This is the end marker
 };
 
@@ -108,6 +110,9 @@ void SlXvSetCurrentState()
 	for (; info->index != XSLFI_NULL; ++info) {
 		_sl_xv_feature_versions[info->index] = info->save_version;
 	}
+	if (MapSizeX() > 8192 || MapSizeY() > 8192) {
+		_sl_xv_feature_versions[XSLFI_EXTRA_LARGE_MAP] = 1;
+	}
 }
 
 /**

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_EXTRA_LARGE_MAP,                        ///< Extra large map
 
 	XSLFI_SIZE,                                   ///< Total count of features, including null feature
 };