Terrain clamping support

[lilleyse]

Extend globe anchors to automatically clamp to the terrain surface as tiles refine. Will likely build off the work in cesium-native:

• Clamp points to terrain cesium-native#72
• Add height-query function to Tileset cesium-native#783

The height query implementation may depend on physics (#153) or RTX ray casting (#154). Unlike metadata picking, we don't need the Fabric prim path, only the intersection point.