Fix vision on plane of air

We can't just unconditionally unblock vision when the cloud moves,
as there could be a temporary poison cloud at that location.

src/mkmaze.c

@@ -1644,7 +1644,7 @@ movebubbles(void)
         for (x = 1; x <= (COLNO - 1); x++)
             for (y = 0; y <= (ROWNO - 1); y++) {
                 levl[x][y] = air_pos;
-                unblock_point(x, y);
+                recalc_block_point(x, y);
                 /* all air or all cloud around the perimeter of the Air
                    level tends to look strange; break up the pattern */
                 xedge = (boolean) (x < gbxmin || x > gbxmax);