def at n
      m = @meta[n]
      return nil if m.nil?
      @io.pos = @pos_of_movi + m[:offset] + 8
      frame = Frame.new(@io.read(m[:size]), m[:id], m[:flag])
      @io.rewind
      frame
    end
def at n
      m = @meta[n]
      return nil if m.nil?
      @io.pos = @pos_of_movi + m[:offset] + 8
      frame = Frame.new(@io.read(m[:size]), m[:id], m[:flag])
      @io.rewind
      frame
    end
Returns one Frame object at the given index.