check if rfbProcessClientInitMessage() should lock cl->sendMutex

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This at least calls rfbTightExtensionInit which calls rfbSendInteractionCaps which has rfbWriteExact.

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Same holds true for rfbProcessClientSecurityType which calls

rfbHandleSecTypeTight
   rfbSendTunnelingCaps
       rfbSendAuthCaps
          rfbProcessClientAuthType
             rfbVncAuthSendChallenge

which has rfbWriteExact.

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OTOH, clientOutput thread will not send anything until auth and init are completed. Or? Investigate!