Refine SEE threshold for capture pruning.

eloDoc suggests that this part of search is worth ~18 Elo.
This patch refines the depth dependence of the SEE threshold.

passed STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 21398 W: 4474 L: 4245 D: 12679
http://tests.stockfishchess.org/tests/view/5abfb0630ebc591a560aae07

passed LTC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 9028 W: 1439 L: 1285 D: 6304
http://tests.stockfishchess.org/tests/view/5abfbff30ebc591a560aae11

Closes https://github.com/official-stockfish/Stockfish/pull/1527

Bench: 6036915