Never hide a mover behind the terrain it sits on

Cesium does not test points, billboards or labels against terrain depth;
only the horizon, 3D tiles and models hide them. Navara compared every
fragment of a marker's quad with the terrain, so markers on slopes or a
little below the rendered terrain disappeared (the flood markers in the
Nepal story). Navara now follows Cesium's rule for movers with the normal
depth test, the contract describes it, and the contract suite checks it.

docs/GLOBE-PRIMITIVES.md

@@ -39,11 +39,15 @@ that has a position and is drawn the same size on screen (or as a model).
 Point instances may also set `outlineColor` independently of the fill,
 so an expanding callout can fade its outline without changing its width.
 
-Instances move every frame without a rebuild. `depthTest` decides whether
-the scene hides them. A pick returns the instance's `pickId` when the set is
-`pickable`. `ready(index?)` says whether a model's glTF or a sprite atlas has
-loaded and `failed(index?)` whether it failed to: a failed set is not loaded
-again, so a layer that retries makes a new set. Cesium loads a model per
+Instances move every frame without a rebuild. With `depthTest: 'normal'`
+(the default), terrain surface depth never hides a mover, even just below
+the surface. The Earth's horizon, 3D tiles and models still occlude it.
+Sprites and points can use `'always'` to draw over the scene, or
+`{ disableWithinM }` to skip depth testing within that camera distance;
+models always use normal depth. A pick returns the instance's `pickId` when
+the set is `pickable`. `ready(index?)` says whether a model's glTF or a sprite
+atlas has loaded and `failed(index?)` whether it failed to: a failed set is
+not loaded again, so a layer that retries makes a new set. Cesium loads a model per
 instance: an index written with another `pickId` loads its own model, so an
 instance moved into a failed index does not inherit the failure.