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

docs/landscape.html

@@ -1054,6 +1054,7 @@
      <li>m1 bits 6..5 : Water class (sea, canal or river)
      <li>m1 bits 4..0: <a href="#OwnershipInfo">owner</a> (for sea, rivers, and coasts normally <tt>11</tt>)</li>
      <li>m2: Depot index (for depots only)</li>
+     <li style="color: blue">m3 bit 0: No flooding state, set if all neighbouring tiles are also water</li>
      <li>m4: Random data for canal or river tiles</li>
      <li>m5: tile type:
       <table>