device-manager-xi2: use allocation for clamping

The coordinates translated by the XI2 device manager were being clamped using
the X window size kept by StageX11. However, when the stage is fullscreen,
that size is not updated to the screen size, but kept the same in order to
allow going back to it when the stage goes out of fullscreen.

https://bugzilla.gnome.org/show_bug.cgi?id=731268