This is part of an ongoing decoupling effort across the codebase and
tidying effort in main.rs. Everything to do with showing the window with
a grid of characters is now managed by the `Display` type. It owns the
window, the font rasterizer, and the renderer. The only info needed from
it are dimensions of characters and the window itself for sizing the
terminal properly. Additionally, the I/O loop has access to wake it up
when new data arrives.
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