Feature Proposal: Thumbnail Preview and Export

[JohnCido]

I was using the motion canvas to create tutorial videos. I find it very useful for my project. But I need to create video thumbnail in image editing software. Another approach is creating a scene, add it to the project, and take a snap.

I think it would be awesome to just create a scene and add it to a separate field in makeProject like this:

export default makeProject({
  ...,
  thumbnail: thumbnailScene,
  scenes: [
    ...
  ],
})

To preview it, maybe we could add a fixed width mark on the timeline at the very beginning. Once clicked, it shows just the first frame of the thumbnail scene. And make the blue play head disappear to indicate that you're previewing the thumbnail.

To export the thumbnail image, maybe add a new export button in video settings, and save the image to ./output/thumbnail.