Support for alpha background

[profejuanca]

Hi, maybe a nice feature is the possibility to change the background color to alpha, because ffmpeg can manage this in some formats (most notably .png sequence) with an option for the background for the script that push a default format for alpha option.

This make possible to add in a video editor for a personalized paper or background color in a layer.

[aspiers]

Great idea! Don't have time to do it myself, but I'd gladly review PRs for this from anyone who's able to help.