Encode layer name in tile factory, remove get_elem()

js/tile_factory.js

@@ -1,5 +1,6 @@
 export class TileFactory {
-    constructor(width, height) {
+    constructor(layer_name, width, height) {
+        this.layer_name = layer_name;
         this.width = width;
         this.height = height;
     }