lomiri-greeter: fix multi-display mode

How to reproduce this issue:

• Install Debian 13 minimal on physical hardware (e.g. a notebook), and run apt install lomiri afterwards. Reboot.
• After reboot, Lomiri Greeter should appear on screen. Attach a second / external display to the test machine now.

What you will observe:

• The external display show Lomiri shell, but without the configured background image like on the primary screen.
• The mouse pointer will be trapped on the external display
• The mouse pointer will not transfer the login prompt to the external display

What should happen:

• Mouse pointer should be movable between displays
• Background image should show up on all screens / displays
• Ideally / optionally: the login prompt follows the mouse pointer (we assume that this code exists in Lomiri Greeter, as the UX design of Unity Greeter 7 was the same, if the code does not exist, skip this item).