Update SEE to return a Value

It seems more natural because the actual returned
value is from PieceValue[] array.

No functional change.

src/movepick.h

@@ -103,7 +103,8 @@ private:
   Move ttMove;
   ExtMove killers[6];
   Square recaptureSquare;
-  int captureThreshold, stage;
+  Value captureThreshold;
+  int stage;
   ExtMove *cur, *end, *endQuiets, *endBadCaptures;
   ExtMove moves[MAX_MOVES];
 };