3dtiles support notes

[malytomas]

3dtiles - json

• nominal resolution - texelSize at finest lod
• defines depth of the finest lod
• contains corners of aabb in physical srs
• extents->ci - childIndex - division strategy for aabb
• content->uri (can be another json or mesh)

implementation notes:
fake mapconfig
json -> free layer
vts meshes
"bc/2-0-2-1.mesh" -> "bc/2-0-2-1-0.jpg" (the last zero is submesh index)
has no surrogates
navigation - externally provided ground plane
may contain a node that has more children but no data in itself