New atmosphere causes banding when combined with bloom post processing

We have an app that has noticeable banding artifacts after upgrading to the latest CesiumJS.


I tracked this back to having bloom enabled with a stepSize other than 1

```js
const viewer = new Cesium.Viewer("cesiumContainer");

const bloom = viewer.scene.postProcessStages.bloom;
bloom.enabled = true;
bloom.uniforms.stepSize = 7;

// Not needed, but makes it even worse and more noticeable
viewer.scene.highDynamicRange = true;
```


This is also noticeable in the Post Processing Sandcastle Example: https://sandcastle.cesium.com/index.html?src=Bloom.html&label=Post%20Processing


Start with the provided reproduction and the Post Processing Sandcastle Bloom example, using bloom.enabled with bloom.uniforms.stepSize set to 7 and optionally highDynamicRange enabled. Compare the rendering with the current CesiumJS version and identify the atmosphere and bloom interaction. Done means the banding artifacts no longer appear with a non-default bloom stepSize.

Written by the indexing model from the issue text.