Use current server time for XUngrabPointer

Similar to 841e9f32d5952d6f16f67294c2da8bf395066a9b but for the
pointer. Fixes a problem where the desktop environment is unresponsive
(to the mouse) after fullscreen exit; Fixes #579.
diff --git a/vncviewer/DesktopWindow.cxx b/vncviewer/DesktopWindow.cxx
index 1bc413d..7652779 100644
--- a/vncviewer/DesktopWindow.cxx
+++ b/vncviewer/DesktopWindow.cxx
@@ -864,7 +864,7 @@
 {
   mouseGrabbed = false;
 #if !defined(WIN32) && !defined(__APPLE__)
-  XUngrabPointer(fl_display, fl_event_time);
+  XUngrabPointer(fl_display, CurrentTime);
 #endif
 }