Saturation is not that good

# Expected Behavior

The saturation makes good results.
(Made with Gimp)

# Current Behavior

The saturation makes no good results.
The colors are quite 'abrupt'.

Original:

# Failure Information (for bugs)

## Steps to Reproduce

```javascript
jimp.read(Buffer.concat(buffers), (error, value) => {
value.color([{apply:"saturate",params:[-70]}],console.log)
value.write("a.png")
});
```

## Context

- Jimp Version: 0.10.2
- Operating System: Kubuntu
- Node version: 10.16.0

## Failure Logs