Extra thinking before accepting draw PVs.

If the PV leads to a draw (3-fold / 50-moves) position
and we're ahead of time, think a little longer, possibly
finding a better way.

As this is most likely effective at higher draw rates,
tried speculative LTC after a yellow STC:

STC:
http://tests.stockfishchess.org/tests/view/59eb173a0ebc590ccbb8975d
LLR: -2.95 (-2.94,2.94) [0.00,5.00]
Total: 56095 W: 10013 L: 9902 D: 36180
elo =    0.688 +-    1.711 LOS:   78.425%

LTC:
http://tests.stockfishchess.org/tests/view/59eba1670ebc590ccbb897b4
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 59579 W: 7577 L: 7273 D: 44729
elo =    1.773 +-    1.391 LOS:   99.381%

bench: 5234652

src/search.cpp

@@ -149,6 +149,7 @@ namespace {
   void update_pv(Move* pv, Move move, Move* childPv);
   void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
   void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
+  bool pv_is_draw(Position& pos);
 
   // perft() is our utility to verify move generation. All the leaf nodes up
   // to the given depth are generated and counted, and the sum is returned.
@@ -475,11 +476,17 @@ void Thread::search() {
               // from the previous search and just did a fast verification.
               const int F[] = { mainThread->failedLow,
                                 bestValue - mainThread->previousScore };
-
               int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
-              double unstablePvFactor = 1 + mainThread->bestMoveChanges;
+
+              Color us = rootPos.side_to_move();
+              bool thinkHard =    DrawValue[us] == bestValue
+                               && Limits.time[us] - Time.elapsed() > Limits.time[~us]
+                               && ::pv_is_draw(rootPos);
+
+              double unstablePvFactor = 1 + mainThread->bestMoveChanges + thinkHard;
 
               bool doEasyMove =   rootMoves[0].pv[0] == easyMove
+                               && !thinkHard
                                && mainThread->bestMoveChanges < 0.03
                                && Time.elapsed() > Time.optimum() * 5 / 44;
 
@@ -1427,6 +1434,24 @@ namespace {
   }
 
 
+  // Is the PV leading to a draw position? Assumes all pv moves are legal
+  bool pv_is_draw(Position& pos) {
+
+    StateInfo st[MAX_PLY];
+    auto& pv = pos.this_thread()->rootMoves[0].pv;
+
+    for (size_t i = 0; i < pv.size(); ++i)
+        pos.do_move(pv[i], st[i]);
+
+    bool isDraw = pos.is_draw(pv.size());
+
+    for (size_t i = pv.size(); i > 0; --i)
+        pos.undo_move(pv[i-1]);
+
+    return isDraw;
+  }
+
+
   // When playing with strength handicap, choose best move among a set of RootMoves
   // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.