Enabling both bloom and ambient occlusion gives incorrect rendering

Enabling both ambient occlusion and bloom results in incorrect rendering, for example when adding `viewer.scene.postProcessStages.ambientOcclusion.enabled = true;` to
the bloom Sandcastle example: https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/?src=Bloom.html&label=Post%20Processing


Start with the Bloom Sandcastle example and the viewer.scene.postProcessStages.ambientOcclusion.enabled setting shown in the issue. Reproduce the rendering with bloom and ambient occlusion enabled together, then inspect the post-process stages involved. Done means the combined effects render correctly without the reported artifact.

Written by the indexing model from the issue text.