Some polygons are rendered incorrectly when fill-opacity is set

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Provided polygon has extra triangular inside when rendered with fill-opacity = 0.3

**mapbox-gl-js version**: both ^2.8.2 and ^1.13.0

**browser**: Chrome

### Steps to Trigger Behavior

See repro link below
1. Create a geojson source
2. Create a layer with following settings
```
type: 'fill',
paint: {
'fill-color': 'rgb(255, 255, 255)',
'fill-opacity': 0.3
}
```
3. Provide geojson

### Link to Demonstration

https://jsfiddle.net/vohtaski/5ztnhcL4/1/

### Expected Behavior

Solid layer is expected as this:

### Actual Behavior
An extra triangular is visible inside polygon

Start with the linked JSFiddle and reproduce the issue using the provided GeoJSON, fill layer settings, and fill-opacity value of 0.3. Compare the result with the expected solid polygon; done means the extra triangular region no longer appears when the polygon is rendered.

Written by the indexing model from the issue text.