PostProcessStageComposite inputPreviousStageTexture doesn't work in some cases

Sandcastle example: https://sandcastle.cesium.com/#c=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

The core code:
```js
const noopShader = `
uniform sampler2D colorTexture;
varying vec2 v_textureCoordinates;
void main()
{
gl_FragColor = texture2D(colorTexture, v_textureCoordinates);
}
`

const stage1 = new Cesium.PostProcessStage({
name: 'stage_1',
fragmentShader: noopShader,
})

const stage211 = new Cesium.PostProcessStage({
name: 'stage_2_1_1',
fragmentShader: `
void main()
{
gl_FragColor = vec4(1.0);
}
`,
})

const stage2121 = new Cesium.PostProcessStage({
name: 'stage_2_1_2_1',
fragmentShader: noopShader,
})
const stage2122 = new Cesium.PostProcessStage({
name: 'stage_2_1_2_2',
fragmentShader: noopShader,
})

const stage212 = new Cesium.PostProcessStageComposite({
name: 'stage_2_1_2',
stages: [stage2121, stage2122],
})

const stage21 = new Cesium.PostProcessStageComposite({
name: 'stage_2_1',
stages: [stage211, stage212],
})
const stage22 = new Cesium.PostProcessStage({
name: 'stage_2_2',
fragmentShader: noopShader,
})

const stage2 = new Cesium.PostProcessStageComposite({
name: 'stage_2',
stages: [stage21, stage22],
inputPreviousStageTexture: false,
})

const stages = viewer.scene.postProcessStages
stages.add(stage1)
stages.add(stage2)
```
Although the `stage2` create with `inputPreviousStageTexture: false`, the `stage22`'s input is the output of the `stage21`, the map turns to white

Browser: Chrome 92.0.4515.159

Operating System: macOS 10.15.7

Start by running the linked Sandcastle example and tracing the PostProcessStageComposite setup for stage2, stage21, and stage22. Read the inputPreviousStageTexture handling in the post-process stage entry points and verify the expected texture flow. Done means stage22 receives stage21's output while stage2 has inputPreviousStageTexture set to false, without the scene turning white.

Written by the indexing model from the issue text.