ui/gtk: Fix relative mouse with multiple monitors

ui/gtk: Fix relative mouse with multiple monitors

To handle relative mouse input the event handler needs to move the mouse
away from the screen edges. Failing to do so results in the mouse
getting stuck at invisible walls. However the current implementation for
this is broken on hosts with multiple monitors.

With multiple monitors the mouse can be located outside of the current
monitor which is not handled by the current code. Also the monitor
itself might be located at coordinates different from (0, 0).

Signed-off-by: Dennis Wölfing <denniswoelfing@gmx.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210720143940.291413-1-denniswoelfing@gmx.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>