[FFmpeg-user] Blending two inputs with custom expression

[Gyan]

Loop the image input i.e. `-loop 1 -i in.png`

Unfortunately, there's a bug at present which prevents the use of
shortest=1 in the blend filter. (https://trac.ffmpeg.org/ticket/6292)

So, you'll have to terminate using -t N where N is the duration of the
video, or if the video has an audio stream, by adding -shortest.