This was only necessary for use with the
avcodec_decode_video2/avcoded_decode_audio4 APIs which are also
deprecated.
Given we use avcodec_send_packet/avcodec_receive_frame, this isn't
necessary, this is even indicated directly within the FFmpeg API changes
document here on 2017-09-26:
https://github.com/FFmpeg/FFmpeg/blob/master/doc/APIchanges#L410
This prevents our code from breaking whenever we update to a newer
version of FFmpeg in the future if they ever decide to fully remove this
API member.
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