What does storyboard-av-mix do?
Muxes per-shot storyboard video with Chinese and English voice-over by retiming video to match VO duration (setpts), writing Video-Chinese/ and Video-English/ (matched by shot id 01, 02, …). Use when the user wants storyboard A/V mix, storyboard A/V mix, VO dubbing mix, video retime to audio, Video-Chinese, Video-English, bilingual VO on cut video, or batch mux Video/ + Chinese/ + English/.