A MetaEisViewport represents an absolute region backend by e.g. a
pointer device. There are two kinds: a standalone viewport, which
corresponds to a viewport that has no neighbours, and a non-standalone,
which represents a region of a global coordinate space.
The reason for having non-standalone viewports is to allow to mirror the
logical monitor layout of a desktop, while the standalone are meant to
represent things that are not part of the logical monitor layout.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3228>