Use H.264 codec first

[tatzyr]

This pull request improves the portability of the video.

Very occasionally, YouTube publishes videos that combine the AV1 codec and MP4 containers.
AV1+MP4 video was downloaded when I specified the quality = "low", format = "video" option (see below).

# tested in youtube-dl 2020.09.20

# Use original 'low' format
$ youtube-dl -f 'worstvideo[ext=mp4]+worstaudio[ext=m4a]/worst[ext=mp4]/worst' DIe_PXlpRd0 --output 'video_%(vcodec)s.%(ext)s'

# => creates video_av01.0.00M.08.0.110.01.01.06.0.f394.mp4

Most podcast players don't support the AV1 codec, so I changed to select H.264 (AVC) codec first.

[mxpv]

Thanks!