windows get shown on top when compton is active

## Current Behavior

Windows appearing after i3lock's start and in combination with `compton` will get shown on top. When `compton` is not active, the behavior is not seen.

Here's a screenshot of a clean Ubuntu VM with the reproduction instructions executed:


## Expected Behavior

Windows shall get drawn below i3lock.

## Reproduction Instructions

```
# On a clean Ubuntu 18.04 installation
$> sudo apt install i3-wm i3lock i3status dunst compton

# And after i3 login:
$> compton -c &
$> dunst &
$> (sleep 7 && notify-send a b) &
$> i3lock
```

## Environment
Output of `i3lock --version`:
```
$> i3lock --version
i3lock: version 2.10 (2017-11-25) © 2010 Michael Stapelberg
```

# Related

#130

Originally reported on dunst-project/dunst#553