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.
You also need to take in account, that if the image is larger, then the mask, the mask itself needs to be scaled up, so it spans over the maximum possible area, like imagemagick does.