avformat: Deprecate AVFMT_FLAG_PRIV_OPT, remove av_demuxer_open on bump

This flag was added in 492026209b9b58eaf6d2ea56423f6b1e1a8a76a5 in conjunction with av_demuxer_open() to allow to pass private options to demuxers. It worked as follows: av_open_input_stream() (the predecessor of avformat_open_input()) would not call the read_header function if this flag is set. Instead the user could set private options of the demuxer via the format's private class after avformat_open_input() and then call av_demuxer_open() which called the format's read_header function. This approach was abandoned in e37f161e66e042d6c2c7470c4d9881df9427fc4a and av_demuxer_open() deprecated; instead the AVDictionary based way of passing private options to the demuxer was choosen. Yet AVFMT_FLAG_PRIV_OPT has never been deprecated and av_demuxer_open() never removed. This commit implements the deprecation of the flag and schedules av_demuxer_open for removal on the next major bump. Given that av_demuxer_open() has been deprecated in 2012 and that this flag is useless without it, the flag will be ignored after the next major version bump.

ARAndreas Rheinhardt committed 5 years ago