state change from READY to PAUSED.
* The element pads are activated in order to receive data in PAUSED. Streaming threads are started. * Some elements might need to return ASYNC and complete the state change when they have enough information. It is a requirement for sinks to return ASYNC and complete the state change when they receive the first buffer or EOS (preroll). Sinks also block the dataflow when in PAUSED. * A pipeline resets the running_time to 0. * Live sources return NO_PREROLL and don't generate data.