Fix Contempt Factor implementation

First disable Contempt Factor during analysis, then
calculate the modified draw score from the point of
view of the player, so from the point of view of
RootPosition color.

Thanks to Ryan Taker for suggesting the fixes.

No functional change.

src/search.cpp

@@ -199,6 +199,8 @@ void Search::think() {
   Position& pos = RootPosition;
   Chess960 = pos.is_chess960();
   Eval::RootColor = pos.side_to_move();
+  Eval::ValueDraw[ Eval::RootColor] = VALUE_DRAW - Eval::ContemptFactor;
+  Eval::ValueDraw[~Eval::RootColor] = VALUE_DRAW + Eval::ContemptFactor;
   TimeMgr.init(Limits, pos.startpos_ply_counter(), pos.side_to_move());
   TT.new_search();
   H.clear();
@@ -538,7 +540,7 @@ namespace {
     {
         // Step 2. Check for aborted search and immediate draw
         if (Signals.stop || pos.is_draw<false>() || ss->ply > MAX_PLY)
-            return Eval::ValueDrawContempt;
+            return Eval::ValueDraw[pos.side_to_move()];
 
         // Step 3. Mate distance pruning. Even if we mate at the next move our score
         // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
@@ -1094,7 +1096,7 @@ split_point_start: // At split points actual search starts from here
 
     // Check for an instant draw or maximum ply reached
     if (pos.is_draw<true>() || ss->ply > MAX_PLY)
-        return Eval::ValueDrawContempt;
+        return Eval::ValueDraw[pos.side_to_move()];
 
     // Decide whether or not to include checks, this fixes also the type of
     // TT entry depth that we are going to use. Note that in qsearch we use