fullscreen-shell: fix black output

Fullscreen-shell forgot to mark the weston_surface as mapped when
mapping the surface and view. With
f962b489 that means no surface from
fullscreen-shell clients is eveer shown. Most notably this broke
screen-share plugin, which is maybe the only "real" user of
fullscreen-shell.

Fix this oversight. Now screen-share works again with RDP-backend.

Fixes: f962b489
	"compositor: Only create paint nodes for mapped surfaces/views"
	(currently unreleased)

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>