LDrawLoader: Improve smooth normal generation performance a bit

[gkjohnson]

Related issue: --

Description

Improves the generation time of smooth normals a bit by doing the following:

• Avoid using Object.keys to get a single element from an object and instead use an immediately terminating for loop.
• Cache index for half edge so we don't have to iterate over the other triangle edges to find a match.
• Convert element vertex / normal storage to use arrays rather than dictionary keys.

With these changes the time to generate smooth normals goes from ~18.25 seconds to ~16 seconds when loading the AT-ST model. Nothing earth shattering but it is an improvement. I also switched to using typed arrays when generating geometry which saves another ~0.5 seconds on the AT-ST total processing time.

Next up I'm planning to bookkeep quads as 4 vertices so we can cut down on trying the smooth the plane diagonal as well as some other performance improvements. I also want to improve the ergonomics of the loader specifically relating to loading models with the LDraw parts library.

cc @yomboprime

[mrdoob]

Thanks!