output-layer: require all layers in wlr_output_state.layers

- Simplifies the backends
- Avoids having two ways to do the same thing: previously one could
  disable a layer by either omitting it from wlr_output_state.layers,
  or by passing a NULL buffer
- We can change our mind in the future: we can allow users to omit
  some layers and define a meaning without breaking the API.

References: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4017#note_1783997

backend/wayland/output.c

@@ -399,31 +399,6 @@ static bool commit_layers(struct wlr_wl_output *output,
 		}
 	}
 
-	// Unmap any layer we haven't seen
-	struct wlr_output_layer *wlr_layer;
-	wl_list_for_each(wlr_layer, &output->wlr_output.layers, link) {
-		bool found = false;
-		for (size_t i = 0; i < layers_len; i++) {
-			if (layers[i].layer == wlr_layer) {
-				found = true;
-				break;
-			}
-		}
-		if (found) {
-			continue;
-		}
-
-		struct wlr_wl_output_layer *layer =
-			get_or_create_output_layer(output, wlr_layer);
-		if (layer == NULL) {
-			continue;
-		}
-
-		// TODO: only do this once
-		wl_surface_attach(layer->surface, NULL, 0, 0);
-		wl_surface_commit(layer->surface);
-	}
-
 	return true;
 }
 

include/wlr/types/wlr_output_layer.h

@@ -36,6 +36,8 @@
  * Callers are responsible for disabling output layers when they need the full
  * output contents to be composited onto a single buffer, e.g. during screen
  * capture.
+ *
+ * Callers must always include the state for all layers on output test/commit.
  */
 struct wlr_output_layer {
 	struct wl_list link; // wlr_output.layers

types/output/output.c

@@ -673,6 +673,11 @@ static bool output_basic_test(struct wlr_output *output,
 	}
 
 	if (state->committed & WLR_OUTPUT_STATE_LAYERS) {
+		if (state->layers_len != (size_t)wl_list_length(&output->layers)) {
+			wlr_log(WLR_DEBUG, "All output layers must be specified in wlr_output_state.layers");
+			return false;
+		}
+
 		for (size_t i = 0; i < state->layers_len; i++) {
 			state->layers[i].accepted = false;
 		}