Improve futility pruning
Adding a small term to the futility calculation that depends on eval - beta.
Refactored to a simpler form.
Passed STC:
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 74176 W: 19323 L: 18954 D: 35899
Ptnml(0-2): 226, 8576, 19117, 8941, 228
https://tests.stockfishchess.org/tests/view/67e6b0946682f97da2178eaf
Passed LTC:
LLR: 2.96 (-2.94,2.94) <0.50,2.50>
Total: 135090 W: 34499 L: 33983 D: 66608
Ptnml(0-2): 79, 14403, 38040, 14969, 54
https://tests.stockfishchess.org/tests/view/67e757cc6682f97da2178f62
closes https://github.com/official-stockfish/Stockfish/pull/5970
Bench: 1875196