Opaque handle for one stream exported by a graph.

Output callbacks receive graph exports in the first argument:

FFix.command(
  "input.mp4",
  fn src -> src[:video] |> FFix.Filter.scale(w: 1280, h: -1) end,
  fn video ->
    FFix.output("out.mp4", video: video, "c:v": :libx264)
  end
)

You normally pass exports to FFix.output/2; constructing this struct directly is not part of the public API.