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/LinearMath/btPoolAllocator.h

@@ -18,6 +18,7 @@ subject to the following restrictions:
 
 #include "btScalar.h"
 #include "btAlignedAllocator.h"
+#include "btThreads.h"
 
 ///The btPoolAllocator class allows to efficiently allocate a large pool of objects, instead of dynamically allocating them separately.
 class btPoolAllocator
@@ -27,6 +28,7 @@ class btPoolAllocator
 	int				m_freeCount;
 	void*			m_firstFree;
 	unsigned char*	m_pool;
+    btSpinMutex     m_mutex;  // only used if BT_THREADSAFE
 
 public:
 
@@ -71,11 +73,16 @@ public:
 	{
 		// release mode fix
 		(void)size;
+        btMutexLock(&m_mutex);
 		btAssert(!size || size<=m_elemSize);
-		btAssert(m_freeCount>0);
+		//btAssert(m_freeCount>0);  // should return null if all full
         void* result = m_firstFree;
-        m_firstFree = *(void**)m_firstFree;
-        --m_freeCount;
+        if (NULL != m_firstFree)
+        {
+            m_firstFree = *(void**)m_firstFree;
+            --m_freeCount;
+        }
+        btMutexUnlock(&m_mutex);
         return result;
 	}
 
@@ -95,9 +102,11 @@ public:
 		 if (ptr) {
             btAssert((unsigned char*)ptr >= m_pool && (unsigned char*)ptr < m_pool + m_maxElements * m_elemSize);
 
+            btMutexLock(&m_mutex);
             *(void**)ptr = m_firstFree;
             m_firstFree = ptr;
             ++m_freeCount;
+            btMutexUnlock(&m_mutex);
         }
 	}