Play turbulent when defending, simpler when attacking

This patch decays a little the evaluation (up to a few percent) for
positions which have a large complexity measure (material imbalance,
positional compensations, etc).

This may have nice consequences on the playing style, as it modifies
the search differently for attack and defense, both effects being
desirable:

- to see the effect on positions when Stockfish is defending, let us
suppose for instance that the side to move is Stockfish and the nnue
evaluation on the principal variation is -100 : this patch will decay
positions with an evaluation of -103 (say) to the same level, provided
they have huge material imbalance or huge positional compensation.
In other words, chaotic positions with an evaluation of -103 are now
comparable in our search tree to stable positions with an evaluation
of -100, and chaotic positions with an evaluation of -102 are now
preferred to stable positions with an evaluation of -100.

- the effect on positions when Stockfish is attacking is the opposite.
Let us suppose for instance that the side to move is Stockfish and the
nnue evaluation on the principal variation is +100 : this patch will
decay the evaluation to +97 if the positions on the principal variation
have huge material imbalance or huge positional compensation. In other
words, stable positions with an evaluation of +97 are now comparable
in our search tree to chaotic positions with an evaluation of +100,
and stable positions with an evaluation of +98 are now preferred to
chaotic positions with an evaluation of +100.

So the effect of this small change of evaluation on the playing style
is that Stockfish should now play a little bit more turbulent when
defending, and choose slightly simpler lines when attacking.

passed STC:
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 268448 W: 68713 L: 68055 D: 131680
Ptnml(0-2): 856, 31514, 68943, 31938, 973
https://tests.stockfishchess.org/tests/view/64e252bb99700912526653ed

passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 141060 W: 36066 L: 35537 D: 69457
Ptnml(0-2): 71, 15179, 39522, 15666, 92
https://tests.stockfishchess.org/tests/view/64e4447a9009777747553725

closes official-stockfish#4762

Bench: 1426295

src/evaluate.cpp

@@ -155,8 +155,9 @@ Value Eval::evaluate(const Position& pos) {
   int material =  pos.non_pawn_material(stm) - pos.non_pawn_material(~stm)
                 + 126 * (pos.count<PAWN>(stm) - pos.count<PAWN>(~stm));
 
-  // Blend optimism with nnue complexity and (semi)classical complexity
+  // Blend optimism and eval with nnue complexity and material imbalance
   optimism += optimism * (nnueComplexity + abs(material - nnue)) / 512;
+  nnue     -= nnue     * (nnueComplexity + abs(material - nnue)) / 32768;
 
   v = (  nnue     * (915 + npm + 9 * pos.count<PAWN>())
        + optimism * (154 + npm +     pos.count<PAWN>())) / 1024;