Globe translucency tile seam artifacts

From https://github.com/CesiumGS/cesium/pull/8726#issuecomment-618777065

Skirts are disabled when the globe is translucent which means you get all the ugly seams that skirts would normally prevent. I considered rendering terrain + skirts to an offscreen texture, then applying translucency, but then you lose all your translucency layers unless you do something expensive like depth peeling. There's also issues with blending this terrain texture with the rest of the scene in non-OIT mode.

[Sandcastle](http://localhost:8080/Apps/Sandcastle/index.html#c=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)

Start with the linked Sandcastle reproduction and compare translucent-globe rendering with skirts enabled. Read the discussion and linked PR #8726 context to establish the intended rendering behavior; done means the visible terrain tile seams are removed without losing translucency layers or introducing the stated blending problems.

Written by the indexing model from the issue text.