TaskSchedule is created and executes on different threads

If you check the GUI version of the Ray Tracer and print the current thread id in the places where a TaskSchedule is created/executed, you will see that different threads create and execute the TaskSchedule. 

I think this is a bug since I don't think sharing TaskSchedules between threads is safe. 

I am not sure, but this could be the cause we sometimes see `clBuildError` with OpenCL devices.