HDR hits the background color


I want the globe on a white background, but if `highDynamicRange` is enabled the background is not white.

```js
var viewer = new Cesium.Viewer('cesiumContainer', {
skyBox: false
});
viewer.scene.backgroundColor = Cesium.Color.clone(Cesium.Color.WHITE);
//viewer.scene.highDynamicRange = false;
```

Start by running the provided Cesium Viewer snippet with skyBox disabled, a white backgroundColor, and highDynamicRange enabled. Trace the rendering entry points related to scene.backgroundColor and scene.highDynamicRange, then verify that the globe renders with a white background while HDR remains enabled.

Written by the indexing model from the issue text.