Release pointer grab when cursor leaves window

We don't need the grab any more if the pointer cannot click on our
window. This makes it possible to shift focus to another application
when we aren't covering all monitors.
diff --git a/vncviewer/DesktopWindow.h b/vncviewer/DesktopWindow.h
index f1bf312..9700044 100644
--- a/vncviewer/DesktopWindow.h
+++ b/vncviewer/DesktopWindow.h
@@ -87,6 +87,8 @@
 
   void grabKeyboard();
   void ungrabKeyboard();
+  void grabPointer();
+  void ungrabPointer();
 
   static void handleGrab(void *data);
 
@@ -123,6 +125,9 @@
   bool delayedFullscreen;
   bool delayedDesktopSize;
 
+  bool keyboardGrabbed;
+  bool mouseGrabbed;
+
   struct statsEntry {
     unsigned fps;
     unsigned pps;