Problem: Visual highlight not working with cursor at end of screen line
and 'showbreak'.
Solution: Only update "vcol_prev" when drawing buffer text.
closes: vim/vim#128658fc6a1dae0
Problem: Using inclusive index for slice is not always desired.
Solution: Add the slice() method, which has an exclusive index. (closes
vim/vim#7408)
6601b62943
Cherry-pick a line in docs added later.
Co-authored-by: Bram Moolenaar <Bram@vim.org>
