Add explicit endgame scaling for opposite bishop positions

[TheRealGioviok]

Passed STC:

Test  | oppbish
Elo   | 4.37 +- 2.84 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.96 (-2.94, 2.94) [0.00, 3.00]
Games | N: 20426 W: 5437 L: 5180 D: 9809
Penta | [286, 2350, 4687, 2601, 289]

https://ob.cwchess.org/test/1446/

Passed LTC:

Test  | oppbish
Elo   | 5.33 +- 3.07 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=64MB
LLR   | 2.96 (-2.94, 2.94) [0.00, 3.00]
Games | N: 14020 W: 3460 L: 3245 D: 7315
Penta | [63, 1614, 3461, 1789, 83]

https://ob.cwchess.org/test/1448/

Bench: 15108078