While renderer as SVG the SaveAsImage png file can't use

### One-line summary [问题简述]
when we use renderer:'svg' to init the echart as following
echarts.init(element[0],theme,{renderer: 'svg'});
the SaveAsImage echart.png file can't use.

### Version & Environment [版本及环境]
+ ECharts version [ECharts 版本]: 4.02
+ Browser version [浏览器类型和版本]: Chrome 63.0.3239.132
+ OS Version [操作系统类型和版本]: Win10

### Expected behaviour [期望结果]

### ECharts option [ECharts配置项]

```javascript
option = {

}

```

### Other comments [其他信息]