Generate tighter bounding region for tile

[baothientran]

The minimum height and maximum height of the regions in the tileset JSON files all seem to be 0.0. The bounding region is implicitly to be the region of the quadtree. However, it is still too large compared to tile content. Some work for this is done in the branch https://github.com/CesiumGS/cdb-to-3dtiles/tree/tight-bound-box