Use xrandr instead of wmctrl

It turns out the Firefox marionette protocol can move/resize the Firefox
window, so `wmctrl` is not needed.  We will need `xrandr`, though, to
help identify screen/monitor geometry, which we will send to the
control service so it knows how to position the Firefox windows.

mkrootfs.sh

@@ -30,9 +30,9 @@ dnf --installroot "${destdir}" install -y \
     openssh-server \
     rsyslog \
     systemd \
-    wmctrl \
     xinit \
     xorg-x11-server-Xorg \
+    xrandr \
     xset \
     zram-generator \
     zram-generator-defaults \