ui: track how much decoded data we consumed when doing SASL encoding
    
    When we encode data for writing with SASL, we encode the entire pending output
    buffer. The subsequent write, however, may not be able to send the full encoded
    data in one go though, particularly with a slow network. So we delay setting the
    output buffer offset back to zero until all the SASL encoded data is sent.
    
    Between encoding the data and completing sending of the SASL encoded data,
    however, more data might have been placed on the pending output buffer. So it
    is not valid to set offset back to zero. Instead we must keep track of how much
    data we consumed during encoding and subtract only that amount.
    
    With the current bug we would be throwing away some pending data without having
    sent it at all. By sheer luck this did not previously cause any serious problem
    because appending data to the send buffer is always an atomic action, so we
    only ever throw away complete RFB protocol messages. In the case of frame buffer
    updates we'd catch up fairly quickly, so no obvious problem was visible.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
    Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
    Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Message-id: 20171218191228.31018-6-berrange@redhat.com
    Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
    (cherry picked from commit 8f61f1c5a6bc06438a1172efa80bc7606594fa07)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>