Problem: `cmdline_show` is emitted unnecessarily each event
loop iteration, because `cmdline_was_last_drawn` is never set.
Solution: Keep track of whether the cmdline was last drawn to avoid
unnecessarily emitting cmdline_show. Set `redraw_state` to
emit `cmdline_pos` when emitting `CursorMovedC`. Only emit
`cmdline_pos` when cmdline was last drawn.