Issue with default value of transcoding resolution

[Winlin]

https://github.com/winlinvip/simple-rtmp-server/issues/169

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[Winlin]

If vwidth and vheight are not filled in during transcoding, the resolution parameters should be ignored and transcoding should be done according to the original resolution.

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[Winlin]

            vbitrate        0;
            vfps            0;
            vwidth          0;
            vheight         0;
            abitrate        0;
            asample_rate    0;
            achannels       0;

When these few parameters are set to 0 (or not supported, as the default is 0), the original video parameters are used, following the source's configuration.

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[Winlin]

After supporting the follow-up source, the simplest configuration for transcoding can be written as follows:

    transcode {
        enabled     on;
        ffmpeg      ./objs/ffmpeg/bin/ffmpeg;
        engine ff {
            enabled         on;
            vcodec          libx264;
            vprofile        main;
            vpreset         medium;
            acodec          libfdk_aac;
            output          rtmp://127.0.0.1:[port]/[app]?vhost=[vhost]/[stream]_[engine];
        }
    }

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