trying to eliminate flicker when rendering screen

Two interesting things:
- We don't really need double-buffering for rendering the screen on the
  sandbox as a progress indicator. Everything else is untouched, and render-screen
  should be doing that as well.

- Rendering just pixels of the fake screen is buttery smooth. It's the
  _graphemes_ that are slowing things down. Even though there's so many
  fewer of them!

As a result, drawing the fake screen less frequently in `evaluate` doesn't
actually help with flicker. Even though it'll make the debug cycle shorter.
So my current plan is to attack flicker in isolation before I mess with
the render frequency.

In this commit I optimized away the cursor handling. Still doesn't seem
to be helping. In fact it actually seems _worse_.

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