x11-toolkits/wlroots: unbreak nesting under X11 after a8bed4f81662

$ pkg install xorg-minimal xterm sway
$ echo "exec xterm" >~/.xinitrc
$ startx
$ sway -c /dev/null
[...]
Assertion failed: (buffer->n_busy > 0), function handle_x11_present_event, file backend/x11/output.c, line 682.

or

$ pkg install cage sway
$ cage env -u WAYLAND_DISPLAY sway -c /dev/null
[...]
Assertion failed: (buffer->n_busy > 0), function handle_x11_present_event, file backend/x11/output.c, line 682.

(cherry picked from commit 9f5b7d7133bace0cdb8e53c6ce836fa78e7d5237)