LDrawLoader example: Add option to merge geometries after loading (#23173)

* GPGPU Protoplanets example: fix pixel trails

* webgl_gpgpu_water.html: Remove mention to orbiting the camera.

* LDrawLoader example: Add option to merge geometries after loading

* Re-enable conditional lines option when merge model option is on

* Remove unnecessary segment vertices permutations

* Fix code format

* Move the mergeObject function into LDrawLoader

* Extract mergeObject() to LDrawUtils.js

* Documentation file for LDrawLoader

* Add comment about using LDrawUtils