mp4 and webm support

[westurner]

STORY: Users can generate .mp4 and/or .webm videos (in addition to .gif)

• https://github.com/sassman/t-rec-rs/blob/main/src/generators/mp4.rs
  • ffmpeg args for .mp4
• https://trac.ffmpeg.org/wiki/Encode/VP9
  • ffmpeg args for .webm

  • libvpx-vp9 is the VP9 video encoder for ​WebM, an open, royalty-free media file format. libvpx-vp9 can save about 20–50% bitrate compared to libx264 (the default H.264 encoder), while retaining the same visual quality.

• gst-launch args for h264
  • https://github.com/MikeWalrus/xdpss/blob/6853b7e45f6cc4870807e693dc32ed680b3718f6/example/stream-pw-node.sh#L21-L24