layers.c: allow layer surfaces on disabled outputs

4f8b80700e added a check for the output
being enabled when creating a layershell surface with automatic output
selection. This causes labwc to send a `layer_surface.close()` and
`wl_surface.destroy()` to layershell clients when creating a new
layershell surface on a disabled output which is still part of the layout
(e.g. `wlopm --off`).

This MR replaces the `output_is_usable()` check with a dedicated check,
bypassing the check for the output being enabled.

Fixes: #3410
Reported-By: coruja

src/layers.c

@@ -687,7 +687,14 @@ handle_new_layer_surface(struct wl_listener *listener, void *data)
 
 	if (!layer_surface->output) {
 		struct output *output = output_nearest_to_cursor(server);
-		if (!output_is_usable(output)) {
+		if (!output || !output->scene_output) {
+			/*
+			 * We are not using output_is_usable() here because
+			 * it also checks if the output is enabled which is
+			 * not the case when wlopm is used. Thus we allow
+			 * layer surfaces to spawn on disabled outputs as
+			 * long as it is part of the scene layout.
+			 */
 			wlr_log(WLR_INFO,
 				"No output available to assign layer surface");
 			wlr_layer_surface_v1_destroy(layer_surface);