Debug: mark tiles where a flood exist

This commit is used for debugging OpenTTD#8492.  Intended to use when starting a new game. Affected tiles will have a sign placed on them, so you can quickly get to them after world generation.

During world generation, town growth algorithm is speed up, allowing the town to perform multiple sequential growth actions, like building houses, roads, and terraforming. If it happens to lower terrain adjacent to sea and then in one of those sequences it builds a road or a house there, it will end up flooded during the "run tile loop" phase of world generation.

src/water_cmd.cpp

@@ -39,6 +39,7 @@
 #include "industry.h"
 #include "water_cmd.h"
 #include "landscape_cmd.h"
+#include "signs_cmd.h"
 
 #include "table/strings.h"
 
@@ -1145,7 +1146,10 @@ void DoFloodTile(TileIndex target)
 		FloodVehicles(target);
 
 		/* flood flat tile */
+		TileType type = GetTileType(target);
+		char *text = type == MP_HOUSE ? "h" : "r";
 		if (Command<CMD_LANDSCAPE_CLEAR>::Do(DC_EXEC, target).Succeeded()) {
+			Command<CMD_PLACE_SIGN>::Do(DC_EXEC, target, text);
 			MakeSea(target);
 			MarkTileDirtyByTile(target);
 			flooded = true;