Smarter opacity handling

src/ol/PluggableMap.js

@@ -39,7 +39,7 @@ import {create as createTransform, apply as applyTransform} from './transform.js
  * @property {boolean} animate
  * @property {import("./transform.js").Transform} coordinateToPixelTransform
  * @property {null|import("./extent.js").Extent} extent
- * @property {Array<*>} declutterItems
+ * @property {Array<DeclutterItems>} declutterItems
  * @property {import("./coordinate.js").Coordinate} focus
  * @property {number} index
  * @property {Array<import("./layer/Layer.js").State>} layerStatesArray
@@ -54,6 +54,13 @@ import {create as createTransform, apply as applyTransform} from './transform.js
  */
 
 
+/**
+ * @typedef {Object} DeclutterItems
+ * @property {Array<*>} items Declutter items of an executor.
+ * @property {number} opacity Layer opacity.
+ */
+
+
 /**
  * @typedef {function(PluggableMap, ?FrameState): any} PostRenderFunction
  */