Use current server time for XUngrabPointer

Similar to 841e9f32d5 but for the
pointer. Fixes a problem where the desktop environment is unresponsive
(to the mouse) after fullscreen exit; Fixes #579.

vncviewer/DesktopWindow.cxx

@@ -864,7 +864,7 @@ void DesktopWindow::ungrabPointer()
 {
   mouseGrabbed = false;
 #if !defined(WIN32) && !defined(__APPLE__)
-  XUngrabPointer(fl_display, fl_event_time);
+  XUngrabPointer(fl_display, CurrentTime);
 #endif
 }