[PATCH] FFmpegWriter: Macro & member cleanup - The `fmt` class member, which was of type AVFormat*, was really just an unnecessary copy of `(AVFormatContext*)oc->oformat`. But we were ASSIGNING into its members, which we were definitely not supposed to be doing. (And in recent FFmpegs, now that `AVFormat` has been `const`d, we can't.) It's gone; now we just use `oc->oformat` anywhere we used to access `fmt`.
- The preprocessor macro to allocate a new _stream_ was a mess of cross purposes: It did allocate a stream, but then it also allocated a new AvCodecCtx on newer FFmpeg releases. Worse (and always galling to me), it proceeded to assign to a variable that WASN'T passed in to the macro, just taking it on faith that it would only be used where that variable was defined. That's just... ugh. So I broke it apart into two steps (stream creation and context allocation), realized the stream creation code was the same for all ffmpeg versions and didn't need to be a macro at all, and now a 4-parameter, 6-line magical macro has been replaced with a simple, zero-side-effect one-liner.
- I also cleaned up the add_video_stream() code to be more like the add_audio_stream() code, since they were bad-different for no discernible reason.
