compositor: Always assign an output when updating a surface transform

Also make all the callers of weston_surface_assign_output() update the
transform instead. This makes sure that when the surface is assigned an
output its bouding box is valid.

This fixes a bug where a newly created surface would have a NULL output
assigned. This would cause weston_surface_schedule_repaint() to not
schedule a repaint, preventing the surface to be shown until something
else caused a repaint.

tests/event-test.c

@@ -49,7 +49,7 @@ handle_surface(struct test_client *client)
 	surface = (struct weston_surface *) resource;
 
 	weston_surface_configure(surface, 100, 100, 200, 200);
-	weston_surface_assign_output(surface);
+	weston_surface_update_transform(surface);
 	weston_surface_set_color(surface, 0.0, 0.0, 0.0, 1.0);
 	wl_list_insert(&layer->surface_list, &surface->layer_link);
 	weston_surface_damage(surface);

tests/text-test.c

@@ -178,7 +178,7 @@ handle_surface(struct test_client *client)
 	surface = (struct weston_surface *) resource;
 
 	weston_surface_configure(surface, 100, 100, 200, 200);
-	weston_surface_assign_output(surface);
+	weston_surface_update_transform(surface);
 	weston_surface_set_color(surface, 0.0, 0.0, 0.0, 1.0);
 
 	data->layer = malloc(sizeof *data->layer);