xwayland: Update root window size when desktop size changes

This fixes grabs on InputOnly windows whose parent is the root window
failing with GrabNotViewable. This is due to window->borderSize/windowSize
being computed as clipped by its parent, resulting in a null region.

Setting up the right size on the root window makes the InputOnly size
correct too, so the GrabNotViewable paths aren't hit anymore.

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 513e3bd3)