A user may have configured an output to be panning, e.g. using xrandr
--output <output> --mode <mode> --panning <size>. Respect this by making
the logical monitor use the panning size, instead of the mode. This
makes e.g. makes the background cover the whole panning size, and panels
etc will cover the whole top of the panned area, instead of just the top
left part covering the monitor if having panned to (0, 0).
No support is added to configuring panning, i.e. a panned monitor
configuration cannot be stored in monitors.xml.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1085
0462208d4e