Commit fcc7501eb8 had the side-effect of
stacking fullscreen windows below docks which went unnoticed since we
don't use docks in GNOME anymore.
Instead of re-introducing the fullscreen layer, which we don't need
otherwise, we can fix this issue by ensuring we stack docks below all
other windows when the monitor they're on is marked fullscreen. This
has the added benefit that the visibility rule for 3rd party docks
becomes the same as gnome-shell's chrome.
https://bugzilla.gnome.org/show_bug.cgi?id=772937
bfb46ff850