When picom is running, slock will show the screenshot of the locked
screen after DPMS changes state to `on'.
https://bbs.archlinux.org/viewtopic.php?id=256547 seems related, but the
suggested fix (adding `no-fading-openclose = true;` to picom's config)
didn't help.
With this commit, the picom service gets "suspended" while the slock
service is running.