window -> global breaks normal behavior

## Issue
Upgrading to latest version of fizzy-ui-utils breaks normal behavior when bundling with parcel's latest version (2.3.2).

When building in production mode
`Uncaught TypeError: e is undefined`

When building in dev mode
No error but slider behaviour is wrong (see image)

As you can see dots disapeared and all slides appart from the 1st one is invisible with flickity-fade

## Reproduction
```
npm i flickity@3.0.0
npm i flickity-as-nav-for@3.0.0
npm i flickity-fade@2.0.0
npm i --save-dev parcel@2.3.2
```
Then init a flickity basic slider with the fade like this
```
let flkty = new Flickity(cells_container, {
lazyLoad: 3,
wrapAround: true,
fade : true,
adaptiveHeight: true,
initialIndex: -1,
prevNextButtons: false,
autoPlay: true
});
```

p.s: I have fixed the `Uncaught TypeError: e is undefined` when building in production by changing the `this` to `global` in fizzy-ui-utils code.

Downgrading to `flickity-fade@1.0.0` fixes the issues but then you need 2 `fizzy-ui-utils` versions

Thanks.

Start by reproducing the issue with flickity@3.0.0, flickity-as-nav-for@3.0.0, flickity-fade@2.0.0, and Parcel 2.3.2 using the configuration shown. Inspect the fizzy-ui-utils code around the `this` versus `global` behavior, then verify that production builds no longer throw and that development sliders retain their dots and visible slides.

Written by the indexing model from the issue text.