Add map bit to suppress water flooding checks

Set if all neighbour tiles are also water
This reduces the overhead of flood checks on large maps

src/saveload/extended_ver_sl.h

@@ -100,6 +100,7 @@ enum SlXvFeatureIndex {
 	XSLFI_INDUSTRY_CARGO_ADJ,                     ///< Industry cargo adjustment patch
 	XSLFI_REALISTIC_TRAIN_BRAKING,                ///< Realistic train braking
 	XSLFI_INFLATION_FIXED_DATES,                  ///< Inflation is applied between fixed dates
+	XSLFI_WATER_FLOODING,                         ///< Water flooding map bit
 
 	XSLFI_RIFF_HEADER_60_BIT,                     ///< Size field in RIFF chunk header is 60 bit
 	XSLFI_HEIGHT_8_BIT,                           ///< Map tile height is 8 bit instead of 4 bit, but savegame version may be before this became true in trunk