Avoid losing updates on partial requests

A request may be for only part of the frame buffer, meaning we cannot
discard all changes just because we've send out an update. There might
still be modified areas remaining that haven't been requested yet.

common/rfb/VNCSConnectionST.cxx

@@ -1056,8 +1056,11 @@ void VNCSConnectionST::writeFramebufferUpdate()
 
     writeRTTPing();
 
+    // The request might be for just part of the screen, so we cannot
+    // just clear the entire update tracker.
+    updates.subtract(requested);
+
     requested.clear();
-    updates.clear();
   }
 
 out: