dynamicsWorldMt: make island mgr aware of whether a parallel solver is present and make handoff from parallel solver to solver-pool more explicit

src/BulletDynamics/Dynamics/btDiscreteDynamicsWorldMt.h

@@ -21,7 +21,6 @@ subject to the following restrictions:
 #include "btSimulationIslandManagerMt.h"
 #include "BulletDynamics/ConstraintSolver/btConstraintSolver.h"
 
-struct InplaceSolverIslandCallbackMt;
 
 ///
 /// btConstraintSolverPoolMt - masquerades as a constraint solver, but really it is a threadsafe pool of them.
@@ -88,7 +87,7 @@ private:
 ATTRIBUTE_ALIGNED16(class) btDiscreteDynamicsWorldMt : public btDiscreteDynamicsWorld
 {
 protected:
-    InplaceSolverIslandCallbackMt* m_solverIslandCallbackMt;
+    btConstraintSolver* m_constraintSolverMt;
 
     virtual void solveConstraints(btContactSolverInfo& solverInfo) BT_OVERRIDE;
 
@@ -126,6 +125,7 @@ public:
 	btDiscreteDynamicsWorldMt(btDispatcher* dispatcher,
         btBroadphaseInterface* pairCache,
         btConstraintSolverPoolMt* constraintSolver,   // Note this should be a solver-pool for multi-threading
+        btConstraintSolver* constraintSolverMt,    // single multi-threaded solver for large islands (or NULL)
         btCollisionConfiguration* collisionConfiguration
     );
 	virtual ~btDiscreteDynamicsWorldMt();