add id incrementing when duplicate material ids are found in scene

src/Materials/material.ts

@@ -661,6 +661,10 @@ export class Material implements IAnimatable {
     constructor(name: string, scene: Scene, doNotAdd?: boolean) {
         this.name = name;
         this.id = name || Tools.RandomId();
+        let idSubscript = 0;
+        while (scene.getMaterialByID(this.id) != null) {
+            this.id = name + "_" + idSubscript;
+        }
 
         this._scene = scene || EngineStore.LastCreatedScene;
         this.uniqueId = this._scene.getUniqueId();