Support multiple sources for layers

src/ol/layer/Layer.js

@@ -57,7 +57,7 @@ import {listen, unlistenByKey} from '../events.js';
  * @typedef {Object} State
  * @property {import("./Layer.js").default} layer Layer.
  * @property {number} opacity Opacity, the value is rounded to two digits to appear after the decimal point.
- * @property {import("../source/State.js").default} sourceState SourceState.
+ * @property {import("../source/Source.js").default|undefined} source Source being rendered (only for multi-source layers).
  * @property {boolean} visible Visible.
  * @property {boolean} managed Managed.
  * @property {import("../extent.js").Extent} [extent] Extent.
@@ -196,6 +196,13 @@ class Layer extends BaseLayer {
     return /** @type {SourceType} */ (this.get(LayerProperty.SOURCE)) || null;
   }
 
+  /**
+   * @return {SourceType} The source being rendered.
+   */
+  getRenderSource() {
+    return this.getSource();
+  }
+
   /**
    * @return {import("../source/State.js").default} Source state.
    */