There're two errors in scrollbar markers in the presence of folded code:

1. Some markers are not displayed (when the marked row numbers are
greater than the total displayed rows count after folding).
2. Code folding / unfolding doesn't trigger markers repainting.

This PR fixes both problems.

Release Notes:

- Fixed scrollbar markers for folded code.

The second problem (markers are repainted after I move the cursor, not
after folding):


https://github.com/zed-industries/zed/assets/2101250/57ed563d-186d-4497-98ab-d4f946416726