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 am ok with moving the method into `EssencePicture` model, but we need to add a check for `picture.nil?` here.