Decal image may not rendered after loading

### Version
5.0.1

### Steps to reproduce
With

```js
var base64 = 'image://data:image/png;base64,...';

option.decal.decals = [{
color: decalColor,
dashArrayX: [[30, 30], [0, 30, 30, 0]],
dashArrayY: [30, 10],
symbol: base64,
symbolSize: 0.8
}]
```

The decal pattern will not be rendered with image at first time, which may have something to do with not calling `dirty` after image loaded.

### What is expected?
Decal pattern renders with image

### What is actually happening?
No decal pattern