More performant way to clamp points to terrain

[kring]

Keep a list of points on (or relative to) the terrain surface as the terrain level-of-detail changes. This will likely work like CesiumJS, where points are associated with tiles and are moved between them as tiles refine/unrefine.

[j9liu]

A "naive" version of this was implemented in CesiumGS/cesium-unreal#1797: the CesiumGlobeAnchor component raycasts every frame and adjusts its terrain-relative height to match the user-specified value.

A more advanced but performant solution would be to ship #1175, and have events for LOD changes that CesiumGlobeAnchor can respond. I'm keeping this issue open for that purpose.