Add: [NewGRF] Map seed as global variable.

This is useful to provide a feature-agnostic, stable random value that differs between games.
One of the possible uses is to e.g. use it to create pseudo-random regions for towns or industries.

src/genworld.cpp

@@ -93,7 +93,6 @@ static void _GenerateWorld()
 		_generating_world = true;
 		if (_network_dedicated) Debug(net, 3, "Generating map, please wait...");
 		/* Set the Random() seed to generation_seed so we produce the same map with the same seed */
-		if (_settings_game.game_creation.generation_seed == GENERATE_NEW_SEED) _settings_game.game_creation.generation_seed = _settings_newgame.game_creation.generation_seed = InteractiveRandom();
 		_random.SetSeed(_settings_game.game_creation.generation_seed);
 		SetGeneratingWorldProgress(GWP_MAP_INIT, 2);
 		SetObjectToPlace(SPR_CURSOR_ZZZ, PAL_NONE, HT_NONE, WC_MAIN_WINDOW, 0);
@@ -302,6 +301,8 @@ void GenerateWorld(GenWorldMode mode, uint size_x, uint size_y, bool reset_setti
 		_settings_game.construction.map_height_limit = std::max(MAP_HEIGHT_LIMIT_AUTO_MINIMUM, std::min(MAX_MAP_HEIGHT_LIMIT, estimated_height + MAP_HEIGHT_LIMIT_AUTO_CEILING_ROOM));
 	}
 
+	if (_settings_game.game_creation.generation_seed == GENERATE_NEW_SEED) _settings_game.game_creation.generation_seed = _settings_newgame.game_creation.generation_seed = InteractiveRandom();
+
 	/* Load the right landscape stuff, and the NewGRFs! */
 	GfxLoadSprites();
 	LoadStringWidthTable();

src/newgrf.cpp

@@ -7046,6 +7046,10 @@ static uint32 GetPatchVariable(uint8 param)
 		case 0x16:
 			return SPR_SHORE_BASE;
 
+		/* Game map seed */
+		case 0x17:
+			return _settings_game.game_creation.generation_seed;
+
 		default:
 			grfmsg(2, "ParamSet: Unknown Patch variable 0x%02X.", param);
 			return 0;