Scale argument in av_video_convert

[stla]

Hello,

I use av to convert some GIFs to MOV files, in order to upload them to Youtube. If the width or the height of the GIF is not divisible by 2, that doesn't work. Therefore it would be nice if we could scale the video by 2. A possibility is:

av_video_convert <- function (video, output = "output.mp4", verbose = TRUE, scale = NULL) 
{
    info <- av_media_info(video)
    if (nrow(info$video) == 0) 
        stop("No suitable input video stream found")
    framerate <- info$video$framerate[1]
    audio <- if (length(info$audio) && nrow(info$audio)) 
        video
    vf <- if(is.null(scale))
        "null"
    else
        sprintf("scale=iw*%s:ih*%s", scale, scale)
    av_encode_video(input = video, audio = audio, output = output, 
        framerate = framerate, vfilter = vf, verbose = verbose)
}

Would you agree to make this change? (or I can do a PR if you want).

[muschellij2]

Related to #31 and #2