This lets the FFmpeg H.264 decoder know when we've reached the end of
the available data, so that it will output the remaining buffered video
frames.
Not sure why it needs this signal to output the buffered frames, but
surely there's a good reason. :^)
We already take locks for the important parts of the data providers'
handle_seek methods, but holding the lock for the entire duration of
the method call prevents cancelling a seek. With this change, locks are
only only held in the thread's loops when updating the loop conditions
and waiting, allowing the seek ID to increase during a seek.
This implementation allows:
- Accurate seeking to an exact timestamp
- Seeking to the keyframe before a timestamp
- Seeking to the keyframe after a timestamp
These three options will be used to satisfy the playback position
selection in the media element's seeking steps.
