Previously, the packet sink push() implementation just set the codec and
notified a wait condition. Then the recorder thread read the codec and
created the AVStream.
But this was racy: an AVFrame could be pushed before the creation of the
AVStream, causing its video_stream_index or audio_stream_index to be
initialized to -1.
Also, in the future, the AVStream initialization might need data
provided by the packet sink open(), so initialize it there (with a
mutex).
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