Call cont_child() more liberally (#3996)
Call cont_child() more liberally (#3996)

Following the reproduction instructions from
https://github.com/i3/i3/issues/3242#issuecomment-436175346

For me, #3242 happened when the following sequence executed:
1. Fullscreening window correctly calls `stop_child()` in
https://github.com/i3/i3/blob/6e24e2ad6f4b1e32cfa27805a3a948d0de50f3b1/i3bar/src/xcb.c#L685
2. Xrandr change, `reconfig_windows()` is called and `output->visible` is
set to `true` in this line:
https://github.com/i3/i3/blob/6e24e2ad6f4b1e32cfa27805a3a948d0de50f3b1/i3bar/src/xcb.c#L1791
3. When the window's fullscreen is disabled,
`handle_visibility_notify()` returns in this line:
https://github.com/i3/i3/blob/6e24e2ad6f4b1e32cfa27805a3a948d0de50f3b1/i3bar/src/xcb.c#L677
because previously `output->visible` was set to `true`

To fix this, I call `cont_child()` more leniently since it is a no-op
when the child is not stopped.

Fixes #3242
Closes #3761
stapelberg/i3,Airblader/i3,Airblader/i3-original,stapelberg/i3,Airblader/i3-original,Airblader/i3-original,i3/i3,i3/i3,stapelberg/i3,Airblader/i3-original,stapelberg/i3,Airblader/i3,Airblader/i3-original,Airblader/i3,i3/i3,Airblader/i3,i3/i3