Terminate the viewer even when waiting for data

It should be possible to exit the viewer even if the network has
stalled in the middle of a transfer.
diff --git a/vncviewer/vncviewer.cxx b/vncviewer/vncviewer.cxx
index 4b989bb..f9258b9 100644
--- a/vncviewer/vncviewer.cxx
+++ b/vncviewer/vncviewer.cxx
@@ -117,6 +117,11 @@
   exitMainloop = true;
 }
 
+bool should_exit()
+{
+  return exitMainloop;
+}
+
 void about_vncviewer()
 {
   fl_message_title(_("About TigerVNC Viewer"));