Decoding: pts of the first frame of the stream in presentation order, in stream time base.
Only set this if you are absolutely 100% sure that the value you set
it to really is the pts of the first frame.
This may be undefined (AV_NOPTS_VALUE).
@note The ASF header does NOT contain a correct start_time the ASF
demuxer must NOT set this.
Decoding: pts of the first frame of the stream in presentation order, in stream time base. Only set this if you are absolutely 100% sure that the value you set it to really is the pts of the first frame. This may be undefined (AV_NOPTS_VALUE). @note The ASF header does NOT contain a correct start_time the ASF demuxer must NOT set this.