Image preview does not work with tmux popup

### Checklist

- [x] I have read through the manual page (`man fzf`)
- [x] I have searched through the existing issues
- [x] For bug reports, I have checked if the bug is reproducible in the latest version of fzf

### Output of `fzf --version`

0.65.0 (04c4269d)

### OS

- [x] Linux
- [ ] macOS
- [ ] Windows
- [ ] Etc.

### Shell

- [x] bash
- [ ] zsh
- [ ] fish

### Problem / Steps to reproduce

Set:
```
FZF_DEFAULT_OPTS="
--multi
--highlight-line
--height 50%
--tmux 80%
--layout reverse
--border sharp
--info inline-right
--preview '$HOME/oss/fzf/bin/fzf-preview.sh {}'
--preview-border line
--tabstop 4"
```
Open `fzf` while inside `tmux`, select any image. (I'm using Konsole, it has builtin sixel support)

Reproduce the issue using the reported FZF_DEFAULT_OPTS, Linux, bash, Konsole, and tmux, then trace the preview and tmux handling from the relevant fzf entry points. Confirm the behavior with an image selection and make sure image previews render correctly inside the tmux popup.

Written by the indexing model from the issue text.