Dither not working

# Expected Behavior
this image
to something like this
# Current Behavior
this image
to this image
there is just nothing happening

# Failure Information (for bugs)

the image is not being dithered

## Steps to Reproduce

my code snippet:
```

Jimp.read('./server/images/example.png', async function (err, image) {

async function dithering() {
console.log('dithering', file)

image.dither565()
image.write(`./server/images/dithered_image.png`)

file = `./server/images/dithered_image.png`
}
//...
//...
```

1. step 1: dither the image
2. step 2: see the result

## Context

- Jimp Version: 0.16.1
- Operating System: Windows 10 64 bit
- Node version: v14.16.0

## Failure Logs
no errors / logs