Use evaluation trend to adjust futility margin

Adjust futility margin in the child node based on whether the
static evaluation is improving.

STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 15271 W: 3157 L: 2958 D: 9156
http://tests.stockfishchess.org/tests/view/5a9f2f8c0ebc590297cb6216

LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 6617 W: 1053 L: 908 D: 4656
http://tests.stockfishchess.org/tests/view/5a9f98390ebc590297cb6241

Ideas for future work:

- Tune the new margins.
- Try to get this idea to work for futility pruning in parent
  nodes as well.

Bench: 5779242

src/search.cpp

@@ -69,7 +69,9 @@ namespace {
   // Razor and futility margins
   const int RazorMargin1 = 590;
   const int RazorMargin2 = 604;
-  Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
+  Value futility_margin(Depth d, bool improving) {
+    return Value((175 - 50 * improving) * d / ONE_PLY);
+  }
 
   // Futility and reductions lookup tables, initialized at startup
   int FutilityMoveCounts[2][16]; // [improving][depth]
@@ -678,6 +680,10 @@ namespace {
                   ss->staticEval, TT.generation());
     }
 
+    improving =   ss->staticEval >= (ss-2)->staticEval
+            /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
+               ||(ss-2)->staticEval == VALUE_NONE;
+
     if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
         goto moves_loop;
 
@@ -702,7 +708,7 @@ namespace {
     // Step 8. Futility pruning: child node (skipped when in check)
     if (   !rootNode
         &&  depth < 7 * ONE_PLY
-        &&  eval - futility_margin(depth) >= beta
+        &&  eval - futility_margin(depth, improving) >= beta
         &&  eval < VALUE_KNOWN_WIN) // Do not return unproven wins
         return eval;