Eye Dome Lighting frustum seam artifact

After https://github.com/AnalyticalGraphicsInc/cesium/pull/8205 is merged there is still a small frustum seam artifact just for point clouds with eye dome lighting. This only happens in multifrustum setups like when log depth is off. EDL only looks at the current frustum's depth texture so it doesn't have enough samples along the frustum seams.

[Sandcastle](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/index.html#c=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)


Reproduce the artifact using the linked Sandcastle example with point clouds, eye dome lighting, multifrustum rendering, and log depth disabled. Investigate how eye dome lighting samples the current frustum's depth texture. Done means the seam artifact is no longer visible in multifrustum setups.

Written by the indexing model from the issue text.