MultiThreaded Demo:

- fixing various race conditions throughout (usage of static vars, etc)
 - addition of a few lightweight mutexes (which are compiled out by default)
 - slight code rearrangement in discreteDynamicsWorld to facilitate multithreading
 - PoolAllocator::allocate() can now be called when pool is full without
     crashing (null pointer returned)
 - PoolAllocator allocate and freeMemory, are OPTIONALLY threadsafe
     (default is un-threadsafe)
 - CollisionDispatcher no longer checks if the pool allocator is full
     before calling allocate(), instead it just calls allocate() and
     checks if the return is null -- this avoids a race condition
 - SequentialImpulseConstraintSolver OPTIONALLY uses different logic in
     getOrInitSolverBody() to avoid a race condition with kinematic bodies
 - addition of 2 classes which together allow simulation islands to be run
   in parallel:
    - btSimulationIslandManagerMt
    - btDiscreteDynamicsWorldMt
 - MultiThreadedDemo example in the example browser demonstrating use of
   OpenMP, Microsoft PPL, and Intel TBB
 - use multithreading for other demos
 - benchmark demo: add parallel raycasting

src/BulletDynamics/Dynamics/btDiscreteDynamicsWorld.h

@@ -30,6 +30,7 @@ class btIDebugDraw;
 struct InplaceSolverIslandCallback;
 
 #include "LinearMath/btAlignedObjectArray.h"
+#include "LinearMath/btThreads.h"
 
 
 ///btDiscreteDynamicsWorld provides discrete rigid body simulation
@@ -68,9 +69,11 @@ protected:
 	bool	m_latencyMotionStateInterpolation;
 
 	btAlignedObjectArray<btPersistentManifold*>	m_predictiveManifolds;
+    btSpinMutex m_predictiveManifoldsMutex;  // used to synchronize threads creating predictive contacts
 
 	virtual void	predictUnconstraintMotion(btScalar timeStep);
 	
+    void integrateTransformsInternal( btRigidBody** bodies, int numBodies, btScalar timeStep );  // can be called in parallel
 	virtual void	integrateTransforms(btScalar timeStep);
 		
 	virtual void	calculateSimulationIslands();
@@ -85,7 +88,9 @@ protected:
 
 	virtual void	internalSingleStepSimulation( btScalar timeStep);
 
-	void	createPredictiveContacts(btScalar timeStep);
+    void releasePredictiveContacts();
+    void createPredictiveContactsInternal( btRigidBody** bodies, int numBodies, btScalar timeStep );  // can be called in parallel
+	virtual void	createPredictiveContacts(btScalar timeStep);
 
 	virtual void	saveKinematicState(btScalar timeStep);