improve the s3m layer

modules/s3m/S3MLayer.js

@@ -6,7 +6,6 @@
 import State from '@dc-modules/state/State'
 import { Layer } from '@dc-modules/layer'
 import S3MTilesLayer from 's3m-lib/S3MTiles/S3MTilesLayer'
-import ClusterLayer from '../layer/type/ClusterLayer'
 
 class S3MLayer extends Layer {
   constructor(id, url, options = {}) {
@@ -19,6 +18,10 @@ class S3MLayer extends Layer {
     return Layer.getLayerType('s3m')
   }
 
+  get readyPromise() {
+    return this._delegate.readyPromise
+  }
+
   _onAdd(viewer) {
     this._viewer = viewer
     delete this._options['context']