Single pixel diffs are not ignored

Using looks-same v3.3.0.

Note that the reference image was generated using WebGL on a Macbook Pro 2015, whereas the generated image is generated using a docker image running Ubuntu with osmesa.

*looks-same configuration*:
```
await looksSameAsync(actual, expected,
{
tolerance: tolerance,
pixelRatio: self._getDPI(), // set to device DPI
ignoreAntialiasing: true,
ignoreCaret: true
});

await looksSame.createDiffAsync({
reference: this._getReferenceFilePath(),
current: this.getActualFilePath(),
diff: this._getDiffFilePath(),
highlightColor: '#ff00ff',
tolerance: tolerance,
pixelRatio: self._getDPI(),
ignoreAntialiasing: true,
ignoreCaret: true
});
```

**reference image**:

**generated image**:

**diff**:

The lower left pixel of the triangle is showing up as different, and the test is failing due to this.