pkg/imgdiff: upper image bounds should be exclusive

The max point of a bounding rectangle for an `image.Image` is exclusive.
So the current loops
```
			for y := i; y <= image1.Bounds().Max.Y; y += cpus {
				for x := 0; x <= image1.Bounds().Max.X; x++ {
```
actually go one past the end. 
Replace both `<=` with `<`.
Luckily `Image.At` doesn't panic in this case, but the code is still technically wrong.