当使用avfilter_graph_parse_ptr接口，传入filters的参数为fps=fps=15;scale=w=960:h=540
ffmpeg就会输出错误：Invalid filterchain containing an unlabelled output pad: "fps=fps=15;scale=w=960:h=540"

说明多filter不能成功连接。
需要对各个filter的输入和输出指定.
因为默认buffersrc为in， buffersink为out
将传入参数修正为：[in]fps=fps=15[framerate];[framerate]scale=w=960:h=540[out]

就能达到将帧率调整为15，分辨率调整为960x540

关联相关代码，如下：

AVFilterInOut *outputs = avfilter_inout_alloc();
outputs->name       = av_strdup("in");
outputs->filter_ctx = buffersrc_ctx;
outputs->pad_idx    = 0;
outputs->next       = NULL;

AVFilterInOut *inputs  = avfilter_inout_alloc();
inputs->name       = av_strdup("out");
inputs->filter_ctx = buffersink_ctx;
inputs->pad_idx    = 0;
inputs->next       = NULL;

char filter_spec[128] = { 0 };
snprintf(filter_spec, sizeof(filter_spec), "[in]%s[framerate];[framerate]%s[out]", "fps=fps=15", "scale=w=960:h=540");

avfilter_graph_parse_ptr(filter_graph, filter_spec,
			&inputs, &outputs, NULL);
avfilter_graph_config(filter_graph, NULL);