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 (<position>)` mask) for `borderLeftWidth`, `borderRightWidth`, etc, for uneven borders, doesn't match the borderRadius of a `<View>`.

**Expected behavior:**

![image](https://user-images.githubusercontent.com/6757532/79366303-da496180-7f43-11ea-920b-08fcb953feff.png)


**Observed behavior:**

(Example with uneven `borderRadius`)

![image](https://user-images.githubusercontent.com/6757532/79366037-65762780-7f43-11ea-9af5-f2263fc30d89.png)

<!-- (Example with even `borderRadius`)

![image](https://user-images.githubusercontent.com/6757532/79366762-a7ec3400-7f44-11ea-8de3-3fd7611f307b.png) -->

<img src="https://user-images.githubusercontent.com/6757532/79390262-4db09a80-7f67-11ea-9516-a4fa687520b5.png" style="width: 200; height: auto;" />




**How to reproduce:**

```jsx
(
  <View
    style={{
      width: 60,
      height: 60,
      borderRadius: 6,
      borderTopLeftRadius: 12,
      borderBottomLeftRadius: 12,
      borderTopRightRadius: 0,
      borderBottomRightRadius: 0,
      borderRightWidth: 0,
      borderLeftWidth: 2,
      borderTopWidth: 2,
      borderBottomWidth: 2,
      borderColor: 'green',
      backgroundColor: 'blue',
    }}
  />
)
```

**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