Move lossless refresh handling to separate method

It makes the logic a bit easier to follow, and also fixes the
case where just a fake update is needed.
diff --git a/common/rfb/VNCSConnectionST.h b/common/rfb/VNCSConnectionST.h
index c992d14..662d9f3 100644
--- a/common/rfb/VNCSConnectionST.h
+++ b/common/rfb/VNCSConnectionST.h
@@ -143,6 +143,7 @@
     void writeFramebufferUpdate();
     void writeNoDataUpdate();
     void writeDataUpdate();
+    void writeLosslessRefresh();
 
     void screenLayoutChange(rdr::U16 reason);
     void setCursor();