Only render what can be realistically displayed to users

`viewport` is unreasonably inefficient for our usecase when the IOTree grows, as it
decides what to display once it rendered to full `IOTree`.
Especially while scrolling, this inefficiency became apparent.

Unfortunately, brick doesn't have any builtin way to improve this, see https://github.com/jtdaugherty/brick/issues/264

We take `drawListElement` as an inspiration, and only render up to
`heightOfBox * 2` elements of the `IOTree`, including expanded tree
nodes.
This speeds up the view noticeably, giving the user a responsive experience.

In particular, we decouple rendering metadata computation from the actual rendering to decide which rows to render.