Support depthFailMaterial for clamped polylines

[OmarShehata]

I couldn't find an issue on this, but it appears that depthFailMaterial is not support for polylines that are clamped to ground. This Sandcastle example works around this issue by not clamping the polyline to ground, and instead sampling the heights of the terrain:

https://sandcastle.cesium.com/index.html?src=Clamp%20to%20Terrain.html

(Click the "sample line positions" in the dropdown).

Here's a version of this that uses clampToGround: true. Notice depthFailMaterial has no effect and there is no warning or anything.

Is this just a matter of threading the depthFailAppearance down into StaticGroundPolylinePerMaterialBatch.js ? If there's a more fundamental reason why clamped polylines can't use depthFailMaterial we should issue a oneTimeWarning explaining that instead of silently failing.