depthTexture packing not working: only contains data in red channel

The depthTexture, when used from a postProcessingStage shader appears to only contain data in the red channel, when depth value should be packed across all channels and unpacked with unpackDepth()

A link to a test in Sandcastle: this outputs the raw depthTexture:

https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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

Start with the linked Sandcastle test, which outputs the raw depthTexture from a postProcessingStage shader. Read how depthTexture is packed and how unpackDepth() consumes it, then reproduce the red-channel-only result in the test. Done means depth values are correctly packed across the expected channels and unpackDepth() returns the intended depth.

Written by the indexing model from the issue text.