LDrawLoader: parallelize parts library downloads to improve load times (#22253)

* Add "setPartsLibraryPath" function

* Add "preloadMaterials" function

* Use single material if possible

* Fix normal smoothing

* Add smarter subobject cache

* Remove static scope stack

* Make "processObject" async

* fix process order

* Fix construction steps

* comments, variable rename

* arrow functions to classic functions

Author: gkjohnson