Pass `DOMRect` to child render function as a second argument

The use case is the need to know which side of the screen the component is going off of. If `TrackVisibility` is already keeping track of a ref and rect, can one or both of these be passed back to the child render function along with `isVisible`?

These are pretty versatile bits of information, I'm sure there are other use cases too.

Additionally, the data might need to be passed to any relevant callbacks like `onVisible` (#39).