xwayland: update the Xwayland screen size first

When updating the overall screen size, Xwayland would first walk the
window tree then update both the xwl_screen and screen size.

As a result, if any ResizeWindow() handler tries to use the xwl_screen
size, it would get the old (wrong) size instead of the new one.

Make sure to update the xwl_screen size first, prior to traverse the
window tree.

This is preparation work for Xwayland fullscreen mode.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>