Adds multithreading support for batch ray casts.

To enable the feature, enable the BULLET2_MULTITHREADING option.

Increases the number of rays that can go in a batch request by storing
them in the shared memory stream instead of the shared memory command.
Adds the API b3RaycastBatchSetNumThreads to specify the number of
threads to use for the raycast batch, also adds the argument numThreads
to the pybullet function rayTestBatch.
Rays are distributed among the threads in a greedy fashion there's a shared
queue of work, once a thread finishes its task, it picks the next
available ray from the task. This works better than pre-distributing the
rays among threads, since there's a large variance in computation time per ray.

Some controversial changes:
- Added a pointer to PhysicsClient to the SharedMemoryCommand struct, this
was necessary to keep the C-API the same for b3RaycastBatchAddRay, while
adding the ray to the shared memory stream instead of the command
struct. I think this may be useful to simplify other APIs as well, that
take both a client handle and a command handle.
- Moved #define SHARED_MEMORY_MAX_STREAM_CHUNK_SIZE to
SharedMemoryPublic. This was necessary for the definition of
MAX_RAY_INTERSECTION_BATCH_SIZE.

examples/SharedMemory/PhysicsDirect.h

@@ -78,6 +78,7 @@ public:
     virtual void setSharedMemoryKey(int key);
 
     void uploadBulletFileToSharedMemory(const char* data, int len);
+    virtual char* getSharedMemoryStreamBuffer();
 
     virtual int getNumDebugLines() const;