Support input concatenation in ffmpeg output mode

[Breakthrough]

In v1.5, support for using ffmpeg to export video files was added. However, there is no support yet for concatenating multiple input videos.

To correctly support this, a temporary list of input files needs to be generated and used with the concat demuxer. The concat filter is not suitable for this purpose, as some operating systems place a hard limit on the allowable length of shell commands.