Pass DOMRect to child render function as a second argument

[liao-frank]

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).