Fix broken logic for sending mouse cursor

The logic got inverted resulting in the cursor being sent needlessly
whenever the cursor moved.
diff --git a/common/rfb/VNCSConnectionST.cxx b/common/rfb/VNCSConnectionST.cxx
index d9bb281..9e58657 100644
--- a/common/rfb/VNCSConnectionST.cxx
+++ b/common/rfb/VNCSConnectionST.cxx
@@ -379,9 +379,10 @@
 {
   if (state() != RFBSTATE_NORMAL) return;
   // Are we switching between client-side and server-side cursor?
-  if (damagedCursorRegion.is_empty() != needRenderedCursor())
+  bool hasRenderedCursor = !damagedCursorRegion.is_empty();
+  if (hasRenderedCursor != needRenderedCursor())
     setCursorOrClose();
-  if (!damagedCursorRegion.is_empty())
+  if (hasRenderedCursor)
     removeRenderedCursor = true;
   if (needRenderedCursor()) {
     updateRenderedCursor = true;