`PerInstanceColorAppearance` shader bug: rendering striations without `flat: true`

(Moved from #7947.)

I think `PerInstanceColorAppearance`'s shaders have a problem: they apply `st` attributes if they're set (for all instances)! `PerInstanceColorAppearance`'s documentation doesn't make note of this at all (though maybe it did in the past, given the documentation fix in #7947; https://github.com/AnalyticalGraphicsInc/cesium/pull/7947/commits/df58cd1f549bda94544850aa0a9008c0aa29884d).


Here is a demonstration; zoom in and rotate the camera about the two ellipses to see the strange rendering effect:

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

Start with the PerInstanceColorAppearance shaders and the linked Sandcastle demonstration, reproducing the striations by zooming and rotating around the two ellipses. Check how st attributes and flat: true are handled, then update the relevant documentation if the behavior remains supported. Done means the rendering behavior is corrected or clearly documented.

Written by the indexing model from the issue text.