added reverse futility pruning

Score of bread_engine_0.0.10_rfp vs bread_engine_0.0.10: 344 - 258 - 231  [0.552] 833
...      bread_engine_0.0.10_rfp playing White: 234 - 62 - 122  [0.706] 418
...      bread_engine_0.0.10_rfp playing Black: 110 - 196 - 109  [0.396] 415
...      White vs Black: 430 - 172 - 231  [0.655] 833
Elo difference: 36.0 +/- 20.1, LOS: 100.0 %, DrawRatio: 27.7 %
SPRT: llr 2.96 (100.6%), lbound -2.94, ubound 2.94 - H1 was accepted

Player: bread_engine_0.0.10_rfp
   "Draw by 3-fold repetition": 197
   "Draw by fifty moves rule": 14
   "Draw by insufficient mating material": 20
   "Loss: Black mates": 62
   "Loss: White mates": 196
   "No result": 5
   "Win: Black mates": 110
   "Win: White mates": 234
Player: bread_engine_0.0.10
   "Draw by 3-fold repetition": 197
   "Draw by fifty moves rule": 14
   "Draw by insufficient mating material": 20
   "Loss: Black mates": 110
   "Loss: White mates": 234
   "No result": 5
   "Win: Black mates": 62
   "Win: White mates": 196
Finished match

src/bread_engine_core.cpp

@@ -283,6 +283,11 @@ float Engine::negamax(int depth, int color, float alpha, float beta){
         if (transposition->best_move != NO_MOVE) sorted_move_gen.set_tt_move(transposition->best_move);
     }
 
+    // reverse futility pruning
+    if (!(pv) && (depth < 5) && (inner_board.evaluate() - static_cast<float>(depth)/10 - 0.2 >= beta)){
+        return beta;
+    }
+
     sorted_move_gen.generate_moves();
     float max_eval = WORST_EVAL;