Record video

[fordhurley]

Add a "Save Video" command for rendering the shader to an mp4 using ffmpeg. This could be great for generating twitter or instagram posts, for example.

I'm a little worried about a can of worms this could open in terms of support issues, but it's probably easier to just call out that this is an experimental feature, put it out there, and see what kinds of problems people run into. I've tested it only on macOS with Homebrew-installed ffmpeg.