def assert_xy!(x, y)
      unless include_xy?(x, y)
        raise ChunkyPNG::OutOfBounds, "Coordinates (#{x},#{y}) out of bounds!"
      end
      true
    end
def assert_xy!(x, y)
      unless include_xy?(x, y)
        raise ChunkyPNG::OutOfBounds, "Coordinates (#{x},#{y}) out of bounds!"
      end
      true
    end
Throws an exception if the x- or y-coordinate is out of bounds.