compositor: when unmapping a surface remove it from the compositor's list

compositor: when unmapping a surface remove it from the compositor's list

compositor.surface_list is recreated every redraw with the mapped
surfaces, but if a surface gets unmapped and then in the same frame
weston_compositor_pick_surface() is called we must make sure it
does not pick the unmapped surface, since it traverses the
surface_list to find one.
If after the unmap the surface gets also deleted it's even more
important, as it must not pick a destroyed surface.