feat(layer): 新增 iframe 弹层在初始加载时的 loading 动画 (#2330)

* feat(layer): 新增 iframe 弹层在初始加载时的 loading 动画

* chore(layer): url 参数添加引号

src/css/modules/layer.css

@@ -12,6 +12,7 @@ html #layuicss-layer{display: none; position: absolute; width: 1989px;}
 .layui-layer-close{position:absolute;}
 .layui-layer-content{position:relative;}
 .layui-layer-border{border: 1px solid #B2B2B2; border: 1px solid rgba(0,0,0,.1); box-shadow: 1px 1px 5px rgba(0,0,0,.2);}
+.layui-layer-load{background:url("data:image/gif;base64,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") #fff center center no-repeat;}
 .layui-layer-setwin span,
 .layui-layer-btn a{display: inline-block; vertical-align: middle; *display: inline; *zoom:1; }