EDL doesn't work with translucent styles

EDL looks great when all alpha values are 1.0:

But it doesn't work at all if any of the points have a transparency:

```js
var viewer = new Cesium.Viewer('cesiumContainer', {
terrainProvider: Cesium.createWorldTerrain()
});

// A ~10 billion point 3D Tileset of the city of Montreal, Canada captured in 2015 with a resolution of 20 cm. Tiled and hosted by Cesium ion.
var tileset = viewer.scene.primitives.add(
new Cesium.Cesium3DTileset({
url: Cesium.IonResource.fromAssetId(28945),
pointCloudShading: {
attenuation: true,
maximumAttenuation: 2
}
})
);

// Fly to a nice overview of the city.
viewer.camera.flyTo({
destination : new Cesium.Cartesian3(1223285.2286828577, -4319476.080312792, 4562579.020145769),
orientation : {
direction : new Cesium.Cartesian3(0.63053223097472, 0.47519958296727743, -0.6136892226931869),
up : new Cesium.Cartesian3(0.7699959023135587, -0.4824455703743441, 0.41755548379407276)
},
easingFunction: Cesium.EasingFunction.QUADRATIC_IN_OUT
});

Sandcastle.addDefaultToolbarButton('Opaque style', function() {
var styleObject = {
color:{
conditions:[
["${Classification} === 1","rgb(47,14,159)"],
["true","rgb(200,200,200)"]
]}};

tileset.style = new Cesium.Cesium3DTileStyle(styleObject);
});

Sandcastle.addToolbarButton('Translucent style', function() {
var styleObject = {
color:{
conditions:[
["${Classification} === 1","rgba(47,14,159,0.8)"],
["true","rgb(200,200,200)"]
]}};

tileset.style = new Cesium.Cesium3DTileStyle(styleObject);
});
```

Start with the supplied Cesium Viewer reproduction using the Montreal 3D Tileset and compare the opaque and translucent style cases. Trace the EDL rendering path for styled point clouds; done means translucent styles render correctly with EDL while preserving the opaque behavior.

Written by the indexing model from the issue text.