Added "-pix_fmt yuv420p" argument to ffmpeg mp4 options for compatibi…

[andrew-klaassen]

…lity with most media players. Wasn't necessary with older ffmpeg version, but with the ffmpeg update the default format has changed from yuv420p to yuv444p, which is not playable on many media players. For details, see:

https://trac.ffmpeg.org/wiki/Encode/H.264

[Herschel]

Thanks! :tada: