libweston-desktop/xdg-shell: update view transforms for xdg popup surfaces

For toplevel surfaces, the shell will do the same thing. Without this, the
commit does not trigger repaints, because the output mask for the surface
is not set.

Without this the popup is not shown unless something else triggers a
repaint. This is usually not seen because the mouse cursor triggers a
repaint at the same time.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>