The shell_surface is (initially) being assigned the default output and
it case it starts on a different ouput, or is being moved there, it it not be
updated to the correct output of the view.

This will break the stacking order of the views by having a fullscreen
view, which will never be demoted a lower stacking order due to newer
non-fullscreen view which will have the shell_surface output set to the
default one. Doing so will ignore the fullscreen view when determining
which view should should be demoted a lower stacking order.

This is only seen in multiple output configuration.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>