xwayland: Use desktop coordinates for pointer events

The Wayland compositor sends pointer coordinates in desktop space, which
can be negative if one output is placed at negative coordinates.

Yet Xwayland would treat those as screen coordinates, which cannot be
negative because relative to the origin.

Queue pointer events from the Wayland compositor as POINTER_DESKTOP
instead of POINTER_SCREEN.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>