Dark Diff

# Expected Behavior

Dark backgrounds when diffing images together instead of the opaque light background

# Current Behavior

The backgrounds still appear light

# Failure Information (for bugs)

A diff with a white opaque background is produced

## Steps to Reproduce

```js
const snapshotA = await Jimp.read(fileA)
snapshotA.background(0x00000000)

const snapshotB = await Jimp.read(fileB)
snapshotB.background(0x00000000)

const diff = Jimp.diff(snapshotA, snapshotB, mismatchThreshold)
```

**Screenshots**

## Context

- Jimp Version: 0.10.3
- Operating System: Mac OS Catalina 10.15.3
- Node version: v10.18.1