Sink elements
-------------

Sink elements consume data. They normally have no source pads.

typical sink elements include:

 - audio/video renderers
 - network sinks
 - filesinks

Sinks are harder to construct than other element types as they are 
treated specially by the GStreamer core.

state changes
-------------

A sink always returns ASYNC from the state change to PAUSED, this 
includes a state change from READY->PAUSED and PLAYING->PAUSED. The
reason for this is that this way we can detect when the first buffer
or event arrives in the sink when the state change completes.

A sink should block on the first event or buffer received in the
PAUSED state before commiting the state to PAUSED.

FLUSHING events have to be handled out of sync with the buffer flow
and take no part in the preroll procedure.

Events other than EOS do not complete the preroll stage.