depth: Limit white 6th move thinking depth to less than 3

c37bd7cc74
Limit white 6th move thinking depth to less than 8
From test result we can known that 3/5/7/8 is OK.

But if set to 8, for this game:

 1.     f4    d2
 2.     b4    d6
 3.     d7    e4
 4.     d3    g4
 5.     a4    c4

White will place at a7 which will lost in 45.
If change to 3/5/7, sometimes place at d5(D), sometimes place at d1(49).

So it's better than 8. Not perfect.

We choose 7.

Now, For this game:

 1.     b4    d2
 2.     f4    d6
 3.     d1    d5
 4.     d7    c4
 5.     g4    e4

5 and 7 is bad move.

Only choose 3, White place at a4/d3 (D).

src/mills.cpp

@@ -476,7 +476,7 @@ Depth get_search_depth(const Position *pos)
             const Depth placingDepthTable9[25] = {
                  +1,  1,  +1,  1,    /* 0 ~ 3 */
                  +3,  3,  +3, 15,    /* 4 ~ 7 */
-                 +5,  2,  +7,  0,    /* 8 ~ 11 */
+                 +5,  2,  +3,  0,    /* 8 ~ 11 */
                  +0,  0,  +0,  0,    /* 12 ~ 15 */
                  +0,  0,  +0,  0,    /* 16 ~ 19 */
                  +0,  0,  +0,  0,    /* 20 ~ 23 */