Dynamic draw value

Try to avoid repetition draws at early midgame,
this should give an edge against weaker opponents
and reduce draw rate.

Tested for regressions with SPRT[-3, 1] and
passed both short TC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 68498 W: 12928 L: 12891 D: 42679

And long TC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 40212 W: 6386 L: 6295 D: 27531

bench: 7990513

src/search.cpp

@@ -206,10 +206,10 @@ void Search::think() {
       }
   }
 
-  if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
+  if (!Options["UCI_AnalyseMode"])
   {
-      int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
-      cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
+      // Dynamic draw value: try to avoid repetition draws at early midgame
+      int cf = std::max(70 - RootPos.game_ply(), 0);
       DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
       DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
   }