Futility tweak

Use a different margin for pruning child nodes.

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 16692 W: 3251 L: 3051 D: 10390
http://tests.stockfishchess.org/tests/view/579b95d10ebc591c761f5c03

LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 24140 W: 3501 L: 3297 D: 17342
http://tests.stockfishchess.org/tests/view/579bb15d0ebc591c761f5c0b

Bench: 7927017

src/search.cpp

@@ -65,7 +65,7 @@ namespace {
 
   // Razoring and futility margin based on depth
   const int razor_margin[4] = { 483, 570, 603, 554 };
-  Value futility_margin(Depth d) { return Value(200 * d); }
+  Value futility_margin(Depth d) { return Value(150 * d); }
 
   // Futility and reductions lookup tables, initialized at startup
   int FutilityMoveCounts[2][16];  // [improving][depth]
@@ -926,7 +926,7 @@ namespace {
 
           // Futility pruning: parent node
           if (   predictedDepth < 7 * ONE_PLY
-              && ss->staticEval + futility_margin(predictedDepth) + 256 <= alpha)
+              && ss->staticEval + 256 + 200 * predictedDepth / ONE_PLY <= alpha)
               continue;
 
           // Prune moves with negative SEE at low depths and below a decreasing