'Return an |_App0Marker| instance for the APP0 marker at *offset* in
*stream*.'

@classmethod
def from_stream(cls, stream, marker_code, offset):

    segment_length = stream.read_short(offset)
    density_units = stream.read_byte(offset, 9)
    x_density = stream.read_short(offset, 10)
    y_density = stream.read_short(offset, 12)
    return cls(marker_code, offset, segment_length, density_units, x_density, y_density)
