Cesium3DTileset. When a tile is first created, its content is not loaded; the content is loaded on-demand when needed based on the view.
Do not construct this directly, instead access tiles through
readonlyboundingSphere : BoundingSphere
readonlychildren : Array.<Cesium3DTile>
readonlycomputedTransform : Matrix4
readonlycontent : Cesium3DTileContent
expireDate : JulianDate
readonlyparent : Cesium3DTile
undefinedif this tile is the root.
When a tile's content points to an external tileset JSON file, the external tileset's
root tile's parent is not
undefined; instead, the parent references
the tile (with its content pointing to an external tileset JSON file) as if the two tilesets were merged.