I have a video with 320 x 240 pixels.
I want to convert it to double size: 640 x 480 pixels.
But I don't want the video upscaled.
I mean: The final file must be a 640 x 480 video file, with the "original video" inside a 320 x 240 rectangle on top left, and the rest of screen filled with black background.
How can I do it with ffmpeg ?