Globe debug bounding volumes

The globe should have an option to show debug bounding volumes for all rendered tiles. Something like `globe.debugShowBoundingVolume` (modeled after 3D Tiles). The Cesium Inspector for terrain can only show one tile bounding volume at a time.

Start by tracing the globe's rendered terrain tiles and the existing Cesium Inspector terrain bounding-volume display, comparing its behavior with the 3D Tiles debug option. Determine where a globe-wide debug setting belongs and verify that enabling it displays bounding volumes for all rendered tiles without changing normal rendering.

Written by the indexing model from the issue text.