Improve the anti-shuffling policy

[snicolet]

We replace the current decrease of the complexity term in initiative
when shuffling by a direct damping of the evaluation. This scheme may
have two benefits over the initiative approach:

a) the damping effect is more brutal for fortresses with heavy pieces
on the board, because the initiative term is almost an endgame term;

b) the initiative implementation had a funny side effect, almost a bug,
in the rare positions where mg > 0, eg < 0 and the tampered eval
returned a positive value (ie with heavy pieces still on the board):
sending eg to zero via shuffling would increase the tampered
eval instead of decreasing it, which is somewhat illogical. This
patch avoids this phenomenon.

How to continue after this patch ? It would be interesting to understand
the effect of storing the evaluations in the transposition tables during
shuffling periods.

STC:
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 43072 W: 8373 L: 8121 D: 26578
Ptnml(0-2): 729, 4954, 9940, 5162, 751
https://tests.stockfishchess.org/tests/view/5ee008ebf29b40b0fc95ade2

LTC:
LLR: 2.94 (-2.94,2.94) {0.25,1.75}
Total: 37376 W: 4816 L: 4543 D: 28017
Ptnml(0-2): 259, 3329, 11286, 3508, 306
https://tests.stockfishchess.org/tests/view/5ee03b06f29b40b0fc95ae0c

Closes #2727

Bench: 4757174

[vondele]

Thanks!

[MichaelB7]

Great patch!