[vim/vim] scroll: window scrolls when using the autocommand window (PR #20884)

```
Problem:  The window scrolls when an autocommand window is used while the
          cursor is behind multi-byte characters.
Solution: Use the byte column instead of the character count when computing
          how many screen lines the text up to the cursor takes.
```


fixes: #12085

---

To reproduce, on a 24 x 80 screen:

    call setline(1, repeat([repeat(nr2char(0x3042), 200)], 20))
    normal! G0100l
    echo line('w0')
    call bufload(bufadd(''))
    echo line('w0')

The topline moves down while the cursor stays where it is.  bufload() is
only used to make Vim use the autocommand window.

plines_win_col() is passed a byte column but consumed one character per
iteration, so with multi-byte characters before the cursor it reported one
screen line too many.  Its only caller is scroll_to_fraction().

The todo.txt entry is kept for the part that is not fixed: the snapshot
restore still resets w_skipcol in a long wrapped line.

You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/20884

-- Commit Summary --

  * scroll: window scrolls when using the autocommand window

-- File Changes --

    M runtime/doc/todo.txt (8)
    M src/misc1.c (5)
    M src/testdir/test_window_cmd.vim (18)

-- Patch Links --

https://github.com/vim/vim/pull/20884.patch
https://github.com/vim/vim/pull/20884.diff

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