Fix: Missing water region invalidation after flooding a half tile with rail in the highest corner

Invalidate water region when any flood occurs.
Convert a few magic numbers to their names.

src/water_cmd.cpp

@@ -1187,6 +1187,7 @@ static void DoFloodTile(TileIndex target)
 		UpdateSignalsInBuffer();
 
 		if (IsPossibleDockingTile(target)) CheckForDockingTile(target);
+		InvalidateWaterRegion(target);
 	}
 
 	cur_company.Restore();