Do not write log depth in vertex shader for upsampled terrain.

[bagnell]

Fixes #6652.

[cesium-concierge]

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[hpinkos]

Perfect, thanks @bagnell!

[lilleyse]

While testing something else I ran into an issue that I think was introduced by this PR. Up close picking doesn't seem to work, at least in the vector tiles sandcastle demo.

[mortac8]

We are seeing black dots on the globe which seems to be related to this fix. We see it with ArcGisImageServerTerrainProvider which we still need and maintain.