Register a callback on the given GstClockID id with the given function and user_data. When passing a GstClockID with an invalid time to this function, the callback will be called immediately with a time set to GST_CLOCK_TIME_NONE. The callback will be called when the time of id has been reached.
The callback func can be invoked from any thread, either provided by the core or from a streaming thread. The application should be prepared for this.