window-x11: Accept any focusable window as fallback focus

As per commit f71151a5 we were ignoring WM_TAKE_FOCUS-only windows as focus
targets, however this might end-up in an infinite loop if there are multiple
non-input windows stacked.

So, accept any focusable window as fallback focus target even if it's a
take-focus one (that might not reply to the request).

Added a stacking test to verify this.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/660
https://gitlab.gnome.org/GNOME/mutter/merge_requests/669

src/tests/meson.build

@@ -114,6 +114,7 @@ stacking_tests = [
   'closed-transient-no-input-no-take-focus-parent',
   'closed-transient-no-input-no-take-focus-parents',
   'closed-transient-no-input-parent',
+  'closed-transient-no-input-parents',
   'closed-transient-no-input-parent-delayed-focus-default-cancelled',
   'minimized',
   'mixed-windows',

src/tests/stacking/closed-transient-no-input-parents.metatest

@@ -0,0 +1,46 @@
+new_client 0 x11
+create 0/1
+show 0/1
+
+new_client 1 x11
+create 1/1
+show 1/1
+
+create 1/2 csd
+set_parent 1/2 1
+accept_focus 1/2 false
+show 1/2
+
+create 1/3 csd
+set_parent 1/3 2
+accept_focus 1/3 false
+show 1/3
+
+create 1/4 csd
+set_parent 1/4 3
+accept_focus 1/4 false
+show 1/4
+
+create 1/5 csd
+set_parent 1/5 3
+show 1/5
+
+wait
+assert_focused 1/5
+assert_stacking 0/1 1/1 1/2 1/3 1/4 1/5
+
+destroy 1/5
+dispatch
+
+assert_focused none
+assert_stacking 0/1 1/1 1/2 1/3 1/4
+
+sleep 250
+assert_focused none
+assert_stacking 0/1 1/1 1/2 1/3 1/4
+
+destroy 1/3
+wait
+
+assert_focused none
+assert_stacking 0/1 1/1 1/2 1/4