osxvideosink: make sure all selectors are performed on the same thread

osxvideosink: make sure all selectors are performed on the same thread

When we are using a dedicated thread to run the main run loop we
must make sure that all selectors are performed on this same thread.
For instance if performSelectorOnMainThread is called from the real
main thread, it will not go through the message queue and will be
executed from the real main thread. By forcing the target thread,
we ensure that all functions will be called either from the real
main thread when the main run loop is running or from our thread
spinning the main loop.