Refactor futility_margin
a small term to the futility calculation that depends on eval - beta.
Passed STC:
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 203136 W: 52797 L: 52239 D: 98100
Ptnml(0-2): 549, 23827, 52255, 24391, 546
https://tests.stockfishchess.org/tests/view/680e84a43629b02d74b15e2e
Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 100356 W: 25950 L: 25507 D: 48899
Ptnml(0-2): 35, 10683, 28302, 11120, 38
https://tests.stockfishchess.org/tests/view/680ebcb03629b02d74b16040
closes https://github.com/official-stockfish/Stockfish/pull/6009
closes https://github.com/official-stockfish/Stockfish/pull/6041
Bench: 1815939
Co-authored-by: Michael Chaly <Vizvezdenec@gmail.com>
Co-authored-by: xu-shawn <50402888+xu-shawn@users.noreply.github.com>