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 calculate two zoom_factors. The first one, ```zoom_x``` is the one that would be used to maintain the "x" dimension when scaling the mask. The second one would maintain the ```y``` dimension. The bigger one of the two will be used to scale both width and height. I think this is what we want.