Fix transient_for endless loop

Other approaches would be:
- Slow/fast pointer technique.
- Using a set/associative map to save 'seen' nodes. i3 does not have
  such data structure.

Counting the total amount of windows is the simpler to implement.

I've also extracted the logic in a function and re-used it in render.c.

Fixes #4404

testcases/t/316-transient-for-loop.t

@@ -0,0 +1,42 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Test that i3 does not get stuck in an endless loop between two windows that
+# set transient_for for each other.
+# Ticket: #4404
+# Bug still in: 4.20-69-g43e805a00
+#
+use i3test i3_config => <<EOT;
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+popup_during_fullscreen smart;
+EOT
+
+my $fs = open_window;
+cmd 'fullscreen enable';
+
+my $w1 = open_window({ dont_map => 1 });
+my $w2 = open_window({ dont_map => 1 });
+
+$w1->transient_for($w2);
+$w2->transient_for($w1);
+$w1->map;
+$w2->map;
+
+does_i3_live;
+
+done_testing;