Not quite ideal: the scrollbar computation only considers
screen_bottom1.line and not screen_bottom1.pos, and so it always assumes
the final line is at the bottom of the screen.
I'm making a deeper change here that I might come to regret. I want to
avoid creating new book-keeping for editor mutations, so I'm putting the
work of computing scrollbar data into clear_screen_line_cache. But that
implies the editor should never clear before updating data, and I caught
one place that wasn't true before. A better name helps avoid this in
future. Let's see how much toil this causes in resolving conflicts.