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/MLCPSolvers/btMLCPSolver.h

@@ -43,6 +43,17 @@ protected:
 	btMLCPSolverInterface* m_solver;
 	int m_fallback;
 
+    /// The following scratch variables are not stateful -- contents are cleared prior to each use.
+    /// They are only cached here to avoid extra memory allocations and deallocations and to ensure
+    /// that multiple instances of the solver can be run in parallel.
+    btMatrixXu m_scratchJ3;
+    btMatrixXu m_scratchJInvM3;
+    btAlignedObjectArray<int> m_scratchOfs;
+    btMatrixXu m_scratchMInv;
+    btMatrixXu m_scratchJ;
+    btMatrixXu m_scratchJTranspose;
+    btMatrixXu m_scratchTmp;
+
 	virtual btScalar solveGroupCacheFriendlySetup(btCollisionObject** bodies, int numBodies, btPersistentManifold** manifoldPtr, int numManifolds,btTypedConstraint** constraints,int numConstraints,const btContactSolverInfo& infoGlobal,btIDebugDraw* debugDrawer);
 	virtual btScalar solveGroupCacheFriendlyIterations(btCollisionObject** bodies ,int numBodies,btPersistentManifold** manifoldPtr, int numManifolds,btTypedConstraint** constraints,int numConstraints,const btContactSolverInfo& infoGlobal,btIDebugDraw* debugDrawer);