lavc: add avcodec_free_context().
Right now, the caller has to manually manage some allocated AVCodecContext fields, like extradata or subtitle_header. This is fragile and prone to leaks, especially if we want to add more such fields in the future. The only reason for this behaviour is so that the AVStream codec context can be reused for decoding. Such reuse is discouraged anyway, so this commit is the first step to deprecating it.
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