Tweak of SEE pruning condition

passed STC
http://tests.stockfishchess.org/tests/view/5d386bda0ebc5925cf0ef49a
LLR: 2.95 (-2.94,2.94) [0.50,4.50]
Total: 56874 W: 12820 L: 12373 D: 31681

passed LTC
http://tests.stockfishchess.org/tests/view/5d38873a0ebc5925cf0ef86e
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 43512 W: 7547 L: 7247 D: 28718

Additional thanks to @locutus2 , @miguel-l and @xoto10 for fruitful discussion.
There may be some more elo there since this tweak was the first one and numbers
are more or less arbitrary.

Closes #2256

Bench 3935523

src/search.cpp

@@ -1041,7 +1041,7 @@ namespace {
  continue;
 
  // Prune moves with negative SEE (~10 Elo)
- if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
+ if (!pos.see_ge(move, Value(-(31 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
  continue;
  }
  else if ( (!givesCheck || !extension)