Mask attributes ignored

`mask` attributes should be applied to a referenced element:
```XML
<defs>
    <polygon id="path-1" points="0 0 0 100 100 100 100 0"/>
</defs>
<mask id="mask-2" fill="white">
    <use xlink:href="#path-1"></use>
</mask>
<polygon fill="red" mask="url(#mask-2)" points="..."></polygon>
```

Expected: red polygon should be displayed
Actual: nothing shown