Fix transcoding example about timebase

[hsnks100]

as-is: The transcoding example does not work with your current testdata/video.mp4.

to-be: It should be made to work.

refer: https://www.ffmpeg.org/doxygen/trunk/transcode_8c-example.html

in official ffmpeg example

/* In this example, we transcode to same properties (picture size,
             * sample rate etc.). These properties can be changed for output
             * streams easily using filters */
            if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
                enc_ctx->height = dec_ctx->height;
                enc_ctx->width = dec_ctx->width;
                enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;
                /* take first format from list of supported formats */
                if (encoder->pix_fmts)
                    enc_ctx->pix_fmt = encoder->pix_fmts[0];
                else
                    enc_ctx->pix_fmt = dec_ctx->pix_fmt;
                /* video time_base can be set to whatever is handy and supported by encoder */
                enc_ctx->time_base = av_inv_q(dec_ctx->framerate); // HERE
            } else {
                enc_ctx->sample_rate = dec_ctx->sample_rate;
                ret = av_channel_layout_copy(&enc_ctx->ch_layout, &dec_ctx->ch_layout);
                if (ret < 0)
                    return ret;
                /* take first format from list of supported formats */
                enc_ctx->sample_fmt = encoder->sample_fmts[0];
                enc_ctx->time_base = (AVRational){1, enc_ctx->sample_rate}; // HERE
            }

I modified your example by referencing the C example.

[asticode]

Thanks for the PR ❤️