Multiple fill overlap

In Pencil.dev & Figma, we can use multiple fills like this:
```json
"fill": [
  { "type": "gradient", "gradientType": "angular", ... },  // 底层
  { "type": "image", "url": "./texture.png", "mode": "fill" }, // 中层
  { "type": "color", "color": "#00000033" }               // 顶层遮罩
]
```