Use ply counter in Position object

And avoid a redundant one passed as argument in
search calls.

Also renamed gamePly in ply to better clarify this
is used as search ply and is set to zero at the
beginning of the search.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/position.h

@@ -100,7 +100,7 @@ enum Phase {
 
 struct StateInfo {
   Key pawnKey, materialKey;
-  int castleRights, rule50, gamePly, pliesFromNull;
+  int castleRights, rule50, ply, pliesFromNull;
   Square epSquare;
   Score value;
   Value npMaterial[2];
@@ -273,7 +273,7 @@ public:
 
   // Game ply information
   int ply() const;
-  void reset_game_ply();
+  void reset_ply();
 
   // Position consistency check, for debugging
   bool is_ok(int* failedStep = NULL) const;
@@ -558,7 +558,7 @@ inline PieceType Position::captured_piece() const {
 }
 
 inline int Position::ply() const {
-  return st->gamePly;
+  return st->ply;
 }
 
 #endif // !defined(POSITION_H_INCLUDED)