Lots of test failures related to the audio decoder (and maybe some other stuff).

Now running ci again.

This is a complete nightmare, with weird breakage all over the place; and it doesn't really seem to be that important, so I'll bump it until post 2.16.

There's a few things that need doing separately, probably:

• some relatively straightforward deprecation warning removals
• the AVCodecContext-related deprecations which seem much trickier
• the new send/receive API
• update the library

Decoding seems ok on new-ffmpeg-take2. But encoding is badly broken after 'switch encoding to FFmpeg send/receive API"; e.g. ffprobe build/test/ffmpeg_encoder_h264_test5.mp4 gives a whole load of errors (and ffcmp does not even try to check video data...!)

It may have something to do with code protected with FF_API_LAVF_AVCTX; or at least something magical that happens if AVStream::codec is set.

One thing seems to be you have to call avcodec_open2 before creating streams... obviously...

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