Increase non-PV IID search depth

bench: 5146380
Merge branch 'master' into increase_iid
Set IID half way between d/2 and d-4

Master IID formula is depth / 2
Previous patch is depth - 4 * ONE_PLY

This one is the middle way:

(dept/2 + depth-4*ONE_PLY)/2  -> depth-2*ONE_PLY-depth/4

After 16000 games at 60+0.05 th 1
ELO: 4.08 +-3.1 (95%) LOS: 99.5%
Total: 16000 W: 2742 L: 2554 D: 10704

bench: 4781239
Double Impact of Gain tables

Very unorthodox idea.

After 16000 games at 60"+0.05
ELO: 3.14 +-3.4 (95%) LOS: 96.6%
Total: 13407 W: 2278 L: 2157 D: 8972

bench: 4705335
Don't early stop if we have a mated score

No functional change.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/search.cpp