shell: Make sure the black surface is opaque

So we don't repaint anything below it.

src/shell.c

@@ -1388,6 +1388,8 @@ create_black_surface(struct weston_compositor *ec,
 	surface->private = fs_surface;
 	weston_surface_configure(surface, x, y, w, h);
 	weston_surface_set_color(surface, 0.0, 0.0, 0.0, 1);
+	pixman_region32_init_rect(&surface->opaque, 0, 0, w, h);
+
 	return surface;
 }