完善加载蒙层

src/core/widget/LoadingMask.js

 class LoadingMask extends Widget {
   constructor() {
     super()
-    this._wrapper = DomUtil.create('div', 'dc-loading-container')
+    this._wrapper = DomUtil.create('div', 'dc-loading-mask')
     this.type = Widget.getWidgetType('loading_mask')
     this._state = State.INITIALIZED
   }

src/core/widget/LocationBar.js

       <span>海拔：${alt.toFixed(2)} 米</span>`
   }
 
+  /**
+   *
+   * @private
+   */
   _cameraHandler() {
     let now = Cesium.getTimestamp()
     if (now < this._lastUpdate + 300) {

src/themes/loading-mask.scss

-.dc-loading-container {
-  width: 100%;
-  height: 100%;
+.dc-loading-mask {
   position: absolute;
-  z-index: 2;
   top: 0;
   left: 0;
+  width: 100%;
+  height: 100%;
   display: flex;
   justify-content: center;
   align-items: center;