Encoders may emit a buffering period SEI without a corresponding
SPS/PPS if the SPS/PPS is carried out-of-band, eg with avcc.
During Annex B conversion, this may result in the SPS/PPS being
inserted *after* the buffering period SEI but before the IDR NAL.
Since the buffering period SEI references the SPS, the SPS/PPS
needs to come first.
Signed-off-by: Anton Khirnov <anton@khirnov.net>
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