Add downscaling option

[EwoutH]

It would be handy to be able to encode in lower resolutions than the input file.

A nice implementation would be to specify a scaling factor (1.5, 2, 3 etc.), a horizontal resolution or vertical resolution. The other two variables would adjust based on the third while keeping the aspect ration. This could be implemented using ffmpeg's -vf scale= (see https://trac.ffmpeg.org/wiki/Scaling) for example.