Nest spatial data structure to optimize additive refinement

[pjcozzi]

This will be an update to tiles.json. It could also impact tile formats since each "sub-title" will need some kind of set of (offset, length) pointers into the tile content.

See How can additive refinement be optimized?.

[ptrgags]

This might be related to the new 3DTILES_multiple_contents extension

[lilleyse]

A spatial subdivision within the content could also accelerate picking and height queries. We'll need to determine if this is worth including in the payload or computing at runtime. CC CesiumGS/cesium#8481

[donmccurdy]

Related:

• Spatial subdivision within a glTF model #553