Non-uniform border mask path overlaps borderRadius

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| Reporting a bug or issue
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The border path polyfill (`Border ()` mask) for `borderLeftWidth`, `borderRightWidth`, etc, for uneven borders, doesn't match the borderRadius of a ``.

**Expected behavior:**


**Observed behavior:**

(Example with uneven `borderRadius`)


**How to reproduce:**

```jsx
(

)
```

**Sketch version:**

`63.1`

**GitHub Project:**

Code that's affected by this issue: https://github.com/elemental-react/primer/blob/918f8f0061e9902ac3bc09e5e28db370e603a912/src/buttons/ButtonGroup.jsx#L12

Can be run from https://github.com/elemental-react/primer/tree/master/examples/sketch

Start with the affected src/buttons/ButtonGroup.jsx entry point and run the example from examples/sketch. Reproduce the shown JSX case with uneven border widths and borderRadius, then trace the border path polyfill used by the rendered View. Done means the non-uniform border mask follows the View's borderRadius as in the expected image.

Written by the indexing model from the issue text.