It's not really manageable to make the fake screen pixel-oriented. Feels
excessive to compare things pixel by pixel when we will mostly be
writing text to screen. It'll also make expected screen assertions
more difficult to manage.
So I'm not sure how to make assertions about pixels for now. Instead
we'll introduce fake screens at draw-grapheme.
This is the right way to be direction-independent. Don't save the cursor
when drawing a single grapheme. Where the next char goes is just a
property of the direction-oriented primitives.
