The small catch is that MtkRegion (and pixman regions) "optimize away"
0-size rectangles, so a 0-sized region will always be seen as having
a 0,0 origin. We don't want that, so transfer the origin separately from
the region.
While at it, make the Wayland pointer lock use one such 0-size region,
to avoid the 1x1px wiggle room that it currently has (accounting for subpixel
motion).
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3420>
The min distance to the right/bottom edge depends on Wayland concepts
(wl_fixed_t) and eventually geometry scale. Move the logic the Wayland
side of the pointer constraints machinery to avoid the backend trying to
figure this out without the proper data.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2460>
