The previous logic tried to keep the position of the top left corner of
the window relative to the top left corner of the monitor. This allowed
the window to move out of the target monitor. This change keeps the
proportions of the distance between the window and the monitor borders
instead if possible. Otherwise it keeps the relative position of the
center of the window clamped to [0,1] to make sure the window lands on
the right output.
This also slightly changes what monitor is considered to be on: the
monitor which contains the center of the window and, if the center is on
no monitor, the monitor wich overlaps the most with the window.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2591>
ec58e74cc5