Do not copy master position in split()

A pointer is enough because after a split point has been
setup master and slaves thread end up calling sp_search() or
sp_search_pv() and here a full copy of split point position is
done again, note that even master does another copy (of itself)
and this is done before any do_move() call so that master Position
is never updated between split() and sp_search().

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

src/thread.h

@@ -46,7 +46,7 @@ const int ACTIVE_SPLIT_POINTS_MAX = 8;
 
 struct SplitPoint {
   SplitPoint *parent;
-  Position pos;
+  const Position* pos;
   SearchStack sstack[THREAD_MAX][PLY_MAX_PLUS_2];
   SearchStack *parentSstack;
   int ply;