Pack the FCI for a Receiver Estimated Maximum Bitrate report.

    https://tools.ietf.org/html/draft-alvestrand-rmcat-remb-03

def pack_remb_fci(bitrate, ssrcs):
    """
    Pack the FCI for a Receiver Estimated Maximum Bitrate report.

    https://tools.ietf.org/html/draft-alvestrand-rmcat-remb-03
    """
    data = b'REMB'
    exponent = 0
    mantissa = bitrate
    while mantissa > 0x3ffff:
        mantissa >>= 1
        exponent += 1
    data += pack('!BBH',
                 len(ssrcs),
                 (exponent << 2) | (mantissa >> 16),
                 (mantissa & 0xffff))
    for ssrc in ssrcs:
        data += pack('!L', ssrc)
    return data