Refactor picture.rb#default_mask

Returns the default centered image mask for a given size.
If the mask is bigger than the image, the mask is scaled down
so the largest possible part of the image is visible.

At this point, we can't know what Jcrop is going to do to all this,
so Jcrop will have to scale down the default_mask to whatever
it needs.
I think, this is not correct:

You shrink the mask, if the mask is larger then the image, but this is a problem, if the image has another ratio, then the mask.

I guess it should be:

```
mask[:width] = image[:width]
mask[:height] = image[:height]
```

But eventually this will still not work, because we tell jCrop to keep the ratio. Maybe a solution is to not tell jCrop to keep the ratio for small images?

Tricky one.