vim: Timer fails to properly update the screen

[Thomas Grainger]

Package: vim
Version: 2:8.0.1766-1
Severity: normal

Dear Maintainer,

```
" reproduce.vim
function! Timer() abort
  call timer_start(0, { _-> matchadd('Search', '.') })
  return ''
endfunction
nnoremap <expr> <c-j> Timer()
```

1. vim  --clean reproduce.vim
2. :so %
3. <c-j>
4. screen is not updated until the cursor is moved

This fails on linux TUI vim. On gVim it works properly. It also works
properly on Windows in gVim and vim.

I think changes in redraw_after_callback from this commit are the cause.

https://github.com/vim/vim/issues/2612

-- Package-specific info:

--- real paths of main Vim binaries ---
/usr/bin/vi is /usr/bin/vim.gtk3
/usr/bin/vim is /usr/bin/vim.gtk3
/usr/bin/gvim is /usr/bin/vim.gtk3
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