wayland: Improve default placement of pointer

Currently, the pointer is moved to the center of the workspace. However,
on a multi-monitor setup, the workspace center may not be the same as
the center of the output it's on. That's the case with my setup -
1920x1080, 1920x1080, 1080x1920 (the monitors are laid out in a row from
left to right).

This change improves the default placement of the pointer by actually
moving it to the center of the screen that contains the workspace center
so the pointer doesn't end up at some "random" position from the user's
point of view.


(cherry picked from commit 4f3ff4fb)