restore isOccluded setter

src/Mesh/babylon.abstractMesh.ts

@@ -134,10 +134,14 @@
         private _occlusionInternalRetryCounter = 0;
 
         protected _isOccluded = false;
-        get isOccluded(): boolean {
+        public get isOccluded(): boolean {
             return this._isOccluded;
         }
 
+        public set isOccluded(value: boolean) {
+            this._isOccluded = value;
+        }
+
         private _isOcclusionQueryInProgress = false;
         public get isOcclusionQueryInProgress(): boolean {
             return this._isOcclusionQueryInProgress;