Adjust pruning constants

This patch is a modification of original tuning done by vondele that failed yellow.
Value differences are divided by 2.

Passed STC
https://tests.stockfishchess.org/tests/view/61d918239fea7913d9c64cdf
LLR: 2.94 (-2.94,2.94) <0.00,2.50>
Total: 98968 W: 26248 L: 25858 D: 46862
Ptnml(0-2): 392, 11085, 26156, 11443, 408

Passed LTC
https://tests.stockfishchess.org/tests/view/61d99e3c9fea7913d9c663e4
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 215232 W: 58191 L: 57492 D: 99549
Ptnml(0-2): 271, 22124, 62138, 22801, 282

closes #3885

bench 4572746

src/search.cpp

@@ -1040,7 +1040,7 @@ namespace {
  continue;
 
  // SEE based pruning (~9 Elo)
- if (!pos.see_ge(move, Value(-218) * depth))
+ if (!pos.see_ge(move, Value(-217) * depth))
  continue;
  }
  else
@@ -1051,15 +1051,15 @@ namespace {
 
  // Continuation history based pruning (~2 Elo)
  if ( lmrDepth < 5
- && history < -3000 * depth + 3000)
+ && history < -3875 * (depth - 1))
  continue;
 
  history += thisThread->mainHistory[us][from_to(move)];
 
  // Futility pruning: parent node (~9 Elo)
  if ( !ss->inCheck
  && lmrDepth < 8
- && ss->staticEval + 142 + 139 * lmrDepth + history / 64 <= alpha)
+ && ss->staticEval + 138 + 137 * lmrDepth + history / 64 <= alpha)
  continue;
 
  // Prune moves with negative SEE (~3 Elo)