Filter root moves filter before copy to threads

Currently root moves are copied to all teh threads
but are DTZ filtered only in main thread at the
beginning of teh search.

This patch moves the TB filtering before the
copy of root moves fixing issue #679

https://github.com/official-stockfish/Stockfish/issues/679

No bench change.

src/thread.cpp

@@ -25,6 +25,7 @@
 #include "search.h"
 #include "thread.h"
 #include "uci.h"
+#include "syzygy/tbprobe.h"
 
 ThreadPool Threads; // Global object
 
@@ -169,7 +170,7 @@ int64_t ThreadPool::nodes_searched() {
 /// ThreadPool::start_thinking() wakes up the main thread sleeping in idle_loop()
 /// and starts a new search, then returns immediately.
 
-void ThreadPool::start_thinking(const Position& pos, StateListPtr& states,
+void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
                                 const Search::LimitsType& limits) {
 
   main()->wait_for_search_finished();
@@ -183,6 +184,8 @@ void ThreadPool::start_thinking(const Position& pos, StateListPtr& states,
           || std::count(limits.searchmoves.begin(), limits.searchmoves.end(), m))
           rootMoves.push_back(Search::RootMove(m));
 
+  Tablebases::filter_root_moves(pos, rootMoves);
+
   // After ownership transfer 'states' becomes empty, so if we stop the search
   // and call 'go' again without setting a new position states.get() == NULL.
   assert(states.get() || setupStates.get());