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

def from_stream(cls, stream, marker_code, offset):
        """
        Return an |_App0Marker| instance for the APP0 marker at *offset* in
        *stream*.
        """
        # field               off  type   notes
        # ------------------  ---  -----  -------------------
        # segment length       0   short
        # JFIF identifier      2   5 chr  'JFIF\x00'
        # major JPEG version   7   byte   typically 1
        # minor JPEG version   8   byte   typically 1 or 2
        # density units        9   byte   1=inches, 2=cm
        # horz dots per unit  10   short
        # vert dots per unit  12   short
        # ------------------  ---  -----  -------------------
        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
        )