make PhysicsClient an interface class, and move shared memory implementation

of physics client into PhysicsClientSharedMemory class/file.
This will allow implementation of UDP and other transport protocols to a physics server.

examples/SharedMemory/PhysicsClientSharedMemory.h

@@ -0,0 +1,48 @@
+#ifndef BT_PHYSICS_CLIENT_SHARED_MEMORY_API_H
+#define BT_PHYSICS_CLIENT_SHARED_MEMORY_API_H
+
+#include "PhysicsClient.h"
+
+//#include "SharedMemoryCommands.h"
+#include "LinearMath/btVector3.h"
+
+class PhysicsClientSharedMemory : public PhysicsClient {
+    struct PhysicsClientSharedMemoryInternalData* m_data;
+
+protected:
+public:
+    PhysicsClientSharedMemory();
+    virtual ~PhysicsClientSharedMemory();
+
+    // return true if connection succesfull, can also check 'isConnected'
+    virtual bool connect();
+
+    virtual void disconnectSharedMemory();
+
+    virtual bool isConnected() const;
+
+    // return non-null if there is a status, nullptr otherwise
+    virtual const struct SharedMemoryStatus* processServerStatus();
+
+    virtual struct SharedMemoryCommand* getAvailableSharedMemoryCommand();
+
+    virtual bool canSubmitCommand() const;
+
+    virtual bool submitClientCommand(const struct SharedMemoryCommand& command);
+
+    virtual int getNumJoints() const;
+
+    virtual void getJointInfo(int index, struct b3JointInfo& info) const;
+
+    virtual void setSharedMemoryKey(int key);
+
+    virtual void uploadBulletFileToSharedMemory(const char* data, int len);
+
+    virtual int getNumDebugLines() const;
+
+    virtual const float* getDebugLinesFrom() const;
+    virtual const float* getDebugLinesTo() const;
+    virtual const float* getDebugLinesColor() const;
+};
+
+#endif  // BT_PHYSICS_CLIENT_API_H