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.