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)