Speed up attack generation for leapers

Speed up attack generation for leapers

chess STC
LLR: 2.98 (-2.94,2.94) [-10.00,5.00]
Total: 2299 W: 535 L: 502 D: 1262
http://www.variantfishtest.org:6543/tests/view/5fd0060c6e23db221d9e94b4

seirawan STC
LLR: 2.95 (-2.94,2.94) [-10.00,5.00]
Total: 896 W: 342 L: 295 D: 259
http://www.variantfishtest.org:6543/tests/view/5fd0ab0f6e23db221d9e94c4

minishogi STC
LLR: 2.96 (-2.94,2.94) [0.00,10.00]
Total: 3418 W: 1785 L: 1633 D: 0
http://www.variantfishtest.org:6543/tests/view/5fd006326e23db221d9e94b7

asean STC
LLR: 2.96 (-2.94,2.94) [0.00,10.00]
Total: 1211 W: 246 L: 174 D: 791
http://www.variantfishtest.org:6543/tests/view/5fd006816e23db221d9e94b9

No functional change.