Points still draw with outline even when outlineWidth = 0

At the entity level, the default outline color is black. In this sandcastle, the point should be invisible, but the outline is still visible. This happens in realistic situations but the bug is obvious to see visually when the fill color is transparent.

Sandcastle:

https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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

```javascript
var entity = viewer.entities.add({
position : Cesium.Cartesian3.fromDegrees(-75.59777, 40.03883),
point : {
pixelSize : 10,
color : Cesium.Color.TRANSPARENT,
outlineWidth : 0
}
});
```


I traced this far enough to suspect that the bug is in the shader code for points, where it mixes the outline color with the fill color regardless of the outline width value.

Start with the linked Sandcastle reproduction and trace the point shader code that mixes the outline and fill colors. Confirm the behavior with a transparent fill and outlineWidth set to 0; done means the point is invisible and no outline remains.

Written by the indexing model from the issue text.