Flush socket before checking buffer

There might be stuff lingering in the buffer simply because flush()
hasn't been called in a while, rather than because the transport is
congested.
diff --git a/common/rfb/VNCSConnectionST.cxx b/common/rfb/VNCSConnectionST.cxx
index 3a072ef..97f7d28 100644
--- a/common/rfb/VNCSConnectionST.cxx
+++ b/common/rfb/VNCSConnectionST.cxx
@@ -832,6 +832,7 @@
   int offset;
 
   // Stuff still waiting in the send buffer?
+  sock->outStream().flush();
   if (sock->outStream().bufferUsage() > 0)
     return true;