Rename RootPosition and shuffle think()

Just slightly code reshuffles, noting interesting here...

No functional change.

src/thread.cpp

@@ -422,7 +422,7 @@ void ThreadPool::start_searching(const Position& pos, const LimitsType& limits,
   Signals.stopOnPonderhit = Signals.firstRootMove = false;
   Signals.stop = Signals.failedLowAtRoot = false;
 
-  RootPosition = pos;
+  RootPos = pos;
   Limits = limits;
   SetupStates = states; // Ownership transfer here
   RootMoves.clear();