Update fullscreen window if changing fullScreenAllMonitors option

vncviewer/DesktopWindow.cxx

     fullscreen_screens(top, bottom, left, right);
   }
 
-  fullscreen();
+  if (!fullscreen_active())
+    fullscreen();
 }
 
 void DesktopWindow::grabKeyboard()
   else
     self->ungrabKeyboard();
 
-  if (fullScreen && !self->fullscreen_active())
+  // Call fullscreen_on even if active since it handles
+  // fullScreenAllMonitors
+  if (fullScreen)
     self->fullscreen_on();
   else if (!fullScreen && self->fullscreen_active())
     self->fullscreen_off();