Add option to scale video to fit in box

[seeeeew]

Most of the time when I rescale a video, I want it to fit inside a box so that the aspect ratio of the original video is kept and neither dimension of the output video is larger than the box I specified. Currently FFmpeg Commander seems to only be able to fit the video to either the specified width or height, but not both at the same time. I'm using -vf "scale=w='min($WIDTH,iw)':h='min($HEIGHT,iw)':force_original_aspect_ratio=decrease" to achieve this. Would be cool if this could be added to the tool as well.

Here's the relevant documentation: https://trac.ffmpeg.org/wiki/Scaling#FittingintoaRectangleStatically-sizedPlayer