def reduce(factor)
      case size
      when IIIF::Image::Size::Percent
        pct = size.percentage * 2**factor
        IIIF::Image::Size::Percent.new(pct)
      else
        size.dup
      end
    end
def reduce(factor)
      case size
      when IIIF::Image::Size::Percent
        pct = size.percentage * 2**factor
        IIIF::Image::Size::Percent.new(pct)
      else
        size.dup
      end
    end
This is used for a second resize by imagemagick after resizing
 by kdu.
 No need to scale most resize operations (only percent)
 @param [Integer] factor to scale by
 @return [IIIF::Image::Size] a copy of self if factor is zero.