Get encoding media properties from Transformer about mediadevices

[lherman-cs]

@at-wat about this issue, I was thinking to create a meta transformer which will return the same data, but it'll listen to changes in the data itself, and will inform the caller through a callback. So, in a sketch, it will like like the following:

// not really sure what to call this..
type VideoPropertyOnChange func(prev, current prop.Media)

func NewVideoPropertyListener(onChange VideoPropertyOnChange) video.TransformFunc {
  var prevProp, currentProp prop.Media
  return video.TransformFunc(func(r video.Reader) video.Reader {
    return func() (img image.Image, err error) {
      img, err = r.Read()
      if err == nil {
        // detect changes here
        // store the currentProp to preProp, and get the currentProp
        onChange(prevProp, currentProp)
      }
      return
    }
  })
}

For getting the first property, the caller just needs to wait for the first callback to get called. Since the data flows after the callback is called, the caller will have a chance to pre-setup things. A useful thing from this property is probably when we want to adjust an encoder. Instead of initializing the encoder manually, we would simply listen to this callback and have a flag to mark the first callback. For the rest of callbacks, it would be just adjusting the existing encoding session.

Overall, I think this approach is somewhat similar to Gstreamer's Autoplugging, https://gstreamer.freedesktop.org/documentation/application-development/advanced/autoplugging.html?gi-language=python.

What do you think about this approach?