(v2) exposing frames

Let’s add an option to expose the FPS-based renderer's frames so that users looking for high performance can work in-step with the renderer. Roughly, I think that would look like:

```go
func EnableFrameMsgs() Cmd

type Frame time.Time
```

@aymanbagabas, we can probably use #1282 as a test to determine if it's indeed a good idea.