Inside a vim8 terminal scrolling on preview window's content is impossible

- [x] I have read through the manual page (`man fzf`)
- [x] I have the latest version of fzf
- [x] I have searched through the existing issues

## Info

- OS
- [x] Linux
- [ ] Mac OS X
- [ ] Windows
- [ ] Etc.
- Shell
- [ ] bash
- [ ] zsh
- [x] fish

## Problem / Steps to reproduce
Hi,

I'm trying to implement my own commands that use fzf in a terminal window.
But I noticed a bug: inside a vim terminal window, it's impossible to scroll the content of the fzf preview window. When I place the cursor inside the preview area and I scroll down fzf catch the event instead of the preview's job. There is no problem inside a "true" terminal or when using the `fzf.vim` plugin without terminal. But the problem also occurs whith fzf.vim commands in terminal mod.
Step to reproduce:
- inside vim `:term`
- then run `fzf --preview 'cat {}'`
- select a long file
- place your cursor inside the preview window's area
- scroll with your mouse/trackpad
- fzf cursor is moved instead of scrolling the content of the preview

:version
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Mar 1 2020 17:21:03)
Patches included : 1-343
Compiled by Arch Linux

Note: the preview scroll remains possible with the binds on `preview-up` and `preview-down`

Reproduce the issue in Vim 8.2 using :term, then run fzf --preview 'cat {}>' with a long file selected. Compare mouse scrolling in the preview area with the existing preview-up and preview-down bindings; done means mouse or trackpad scrolling changes the preview content instead of moving the fzf cursor.

Written by the indexing model from the issue text.