LDrawLoader: Improve parts library ergonomics, improve normal smoothing functionality

[gkjohnson]

Related issue: --

Description

• Change the loader so meshes only receive a single material instead of an array if only one material is present.
• Add "setPartsLibraryPath" function (see here)
• Add async "preloadMaterials" function so color files can be preloaded and color codes seeded before loading a model. Without preloading color codes most "unpacked" models don't seem to load.
• Improve the normal smoothing to handle cases where common edges were not getting normals merged by using a "reference"-like structure with the normals wrapped in an object.

Here are the types of normal issues that have been fixed:

BEFORE	AFTER

cc @yomboprime

In the next PR I'll be working to parallelize the downloads when using the parts library to improve load times.

[yomboprime]

Just an insignificant topic: in the function parseColourMetaDirective, the first comment says the function returns null on error. But it never returns such a value (errors are reported through throw)

[gkjohnson]

the first comment says the function returns null on error. But it never returns such a value (errors are reported through throw)

Thanks! I think that's okay but I'll update that comment in the upcoming PR, then.

[mrdoob]

Thanks!