add ClearDraw

sources/Box2D/Common/b2Draw.h

@@ -79,6 +79,8 @@ public:
 	/// @param xf a transform.
 	virtual void DrawTransform(const b2Transform& xf) = 0;
 
+    /// Clear draw commands
+    virtual void ClearDraw() = 0;
 protected:
 	uint32 m_drawFlags;
 };

sources/Box2D/Dynamics/b2World.cpp

@@ -1139,6 +1139,8 @@ void b2World::DrawDebugData()
 		return;
 	}
 
+    g_debugDraw->ClearDraw();
+    
 	uint32 flags = g_debugDraw->GetFlags();
 
 	if (flags & b2Draw::e_shapeBit)