Instead of tracking in-flight seeks across all the sinks in the seeking
state handler, move the logic to PlaybackManager to determine the
overall status and then notify the state of that status to potentially
trigger resumption.
The buffering state handler can then share essentially the same logic
instead of having the playback manager specifically track the blocked
tracks for it.
This is an intermediate step towards unifying the pipeline around new
Producer/Sink interfaces. Producers now have a pull() method that gets
the next piece of data from them. The pull() method returns a status
that can indicate whether it has current data, and if not, why it's
unavailable. This signal will be passed down the pipeline to the final
sink, which can expose the signal to its user, which in the normal
playback pipeline is PlaybackManager. The signal can be used to
transition between playback states. Currently, this is only hooked up
to the buffering state, but should be used later for ending playback
as well as decoding error propagation.
Buffering is now determined solely based on whether the pipeline is
blocked on incomplete data, so the ready state for video now progresses
past HAVE_METADATA immediately after playback manager initializes. This
will change when files have buffered ranges.
This state will indicate to the media element that it's not guaranteed
to have a frame yet, for the purposes of determining the ready state.
JavaScript should be sure that video elements with a ready state of
HAVE_CURRENT_DATA or greater represent the current video frame already.
To allow the state to be exited if audio is disabled, audio tracks are
now only added to the buffering set on enable if the audio sink exists,
since without the sink starting the data provider, it will never be
removed.
This is a step towards making video ref tests.
