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/BulletCollision/BroadphaseCollision/btDbvt.h

@@ -267,7 +267,6 @@ struct	btDbvt
 
 	
 	btAlignedObjectArray<sStkNN>	m_stkStack;
-	mutable btAlignedObjectArray<const btDbvtNode*>	m_rayTestStack;
 
 
 	// Methods
@@ -357,6 +356,7 @@ struct	btDbvt
 								btScalar lambda_max,
 								const btVector3& aabbMin,
 								const btVector3& aabbMax,
+                                btAlignedObjectArray<const btDbvtNode*>& stack,
 								DBVT_IPOLICY) const;
 
 	DBVT_PREFIX
@@ -1006,7 +1006,8 @@ inline void		btDbvt::rayTestInternal(	const btDbvtNode* root,
 								btScalar lambda_max,
 								const btVector3& aabbMin,
 								const btVector3& aabbMax,
-								DBVT_IPOLICY) const
+                                btAlignedObjectArray<const btDbvtNode*>& stack,
+                                DBVT_IPOLICY ) const
 {
         (void) rayTo;
 	DBVT_CHECKTYPE
@@ -1016,7 +1017,6 @@ inline void		btDbvt::rayTestInternal(	const btDbvtNode* root,
 
 		int								depth=1;
 		int								treshold=DOUBLE_STACKSIZE-2;
-		btAlignedObjectArray<const btDbvtNode*>&	stack = m_rayTestStack;
 		stack.resize(DOUBLE_STACKSIZE);
 		stack[0]=root;
 		btVector3 bounds[2];