Explicitly clean up grabs on disconnect

They are not automatically cleaned up with the window on Windows, and we
might want to reconnect so we cannot rely on the process cleanup either.

vncviewer/DesktopWindow.cxx

@@ -245,6 +245,11 @@ DesktopWindow::DesktopWindow(int w, int h, const char *name,
 
 DesktopWindow::~DesktopWindow()
 {
+  // Don't leave any dangling grabs as they are not automatically
+  // cleaned up on all platforms
+  ungrabPointer();
+  ungrabKeyboard();
+
   // Unregister all timeouts in case they get a change tro trigger
   // again later when this object is already gone.
   Fl::remove_timeout(handleGrab, this);